in_dummy plugin

This pull request is aimed for importing https://github.com/tagomoris/fluent-plugin-dummydata-producer to fluentd itself. Importing this plugin helps us to show examples of config in ML or docs easily, and to test fluentd and plugins manually without fluent-cat.

## Difference with original

1. Renamed to `in_dummy` plugin
2. `dummydata#{num}` parameter is now `dummy` parameter which accepts JSON hash or Array of JSON hash. 
3. Remove dependency with `fluent-mixin-config-placeholders`
4. [dummer](https://github.com/sonots/dummer/blob/5b22ad5b430ef297c52f3327e32506bbcea7d8a4/lib/dummer/worker.rb#L38-L52) is also a reference.

## Usage

```
<source>
  type dummy
  tag  dummy.data
  rate 500        # messages per second
  dummy  [
    {"type":"sample","code":50,"format":"json string allowed"},
    {"message":"other format needed?"},
    {"comment":"N of dummydataN is number and not limited"}
  ]
</source>
```

With this configuration, this plugin emits about 500 messages in 1 second (0 -> 1 -> 2 -> 0 -> ...).

NOTE: multiline array format can be used only with --use-v1-format. With --use-v0-format, `dummy` parameter must be written in one line.

If you need message id for acknowledgement tests, incremental id feature available.

```
<source>
  type dummy
  tag  dummy.data
  rate 500        # messages per second
  dummy {"type":"sample","code":50,"format":"json string allowed"}
  auto_increment_key id
</source>
```
Dummy event generation can be done in `configure` instead of `emit`, which is called highly freaquently.
But it is not so serious because this plugin is for tests.