Resource create/update should be able to handle yaml

Fixes: https://github.com/StackStorm/st2/issues/822

## What?

CLI commands should be able to take in YAML files and be able to create resources. 

## Pending

There is a -j option which prints the resources in JSON. Since we support only JSON in APIs, I decided not to support a -y option for printing YAML. However, this leads to an inconsistency.
If we decide to support a -y option, I'll open another PR. 
 
## Example: 

```
(virtualenv)~/s/c/e/rules git:master ❯❯❯ st2 rule create sample_rule_with_webhook.yaml                                                                                                                                                                                                                    ⏎ ✭ ✱ ◼
+-------------+---------------------------------------------------------+
| Property    | Value                                                   |
+-------------+---------------------------------------------------------+
| id          | 5474e81d0640fd21f1d73192                                |
| name        | examples.webhook_file                                   |
| description | Sample rule dumping webhook payload to a file.          |
| action      | {                                                       |
|             |     "ref": "core.local",                                |
|             |     "parameters": {                                     |
|             |         "cmd": "echo \"{{trigger.body}}\" >>            |
|             | /tmp/st2.webhook_sample.out"                            |
|             |     }                                                   |
|             | }                                                       |
| criteria    | {                                                       |
|             |     "trigger.body.name": {                              |
|             |         "pattern": "st2",                               |
|             |         "type": "equals"                                |
|             |     }                                                   |
|             | }                                                       |
| enabled     | True                                                    |
| trigger     | {                                                       |
|             |     "type": "core.st2.webhook",                         |
|             |     "parameters": {                                     |
|             |         "url": "sample"                                 |
|             |     },                                                  |
|             |     "pack": "core"                                      |
|             | }                                                       |
+-------------+---------------------------------------------------------+
```

Now get the rule via CLI. 

```
(virtualenv)~/s/c/e/rules git:master ❯❯❯ st2 rule get examples.webhook_file                                                                                                                                                                                                                                 ✭ ✱ ◼
+-------------+---------------------------------------------------------+
| Property    | Value                                                   |
+-------------+---------------------------------------------------------+
| id          | 5474e81d0640fd21f1d73192                                |
| name        | examples.webhook_file                                   |
| description | Sample rule dumping webhook payload to a file.          |
| action      | {                                                       |
|             |     "ref": "core.local",                                |
|             |     "parameters": {                                     |
|             |         "cmd": "echo \"{{trigger.body}}\" >>            |
|             | /tmp/st2.webhook_sample.out"                            |
|             |     }                                                   |
|             | }                                                       |
| criteria    | {                                                       |
|             |     "trigger.body.name": {                              |
|             |         "pattern": "st2",                               |
|             |         "type": "equals"                                |
|             |     }                                                   |
|             | }                                                       |
| enabled     | True                                                    |
| trigger     | {                                                       |
|             |     "type": "core.st2.webhook",                         |
|             |     "parameters": {                                     |
|             |         "url": "sample"                                 |
|             |     },                                                  |
|             |     "pack": "core"                                      |
|             | }                                                       |
+-------------+---------------------------------------------------------+
(virtualenv)~/s/c/e/rules git:master ❯❯❯
```
Should probably just say "Unsupported file type".