Show collection tip


If a user adds a page to a directory beginning with and underscore, they're probably working with collections.
Octopress should check Jekyll's configuration to see if they've configured the collection and if they haven't,
it should output a block of configuration, instructing them how to add the new collection.

Here's an example command:

```
$ octopress new page _parkr_rulez/reason2
```

and the output it would generate:

```
New page: _parkr_rulez/reason2.html

TIP: To create a new 'parkr_rulez' collection, add this to your Jekyll configuration
----------------
collections:
  parkr_rulez:
    output: true
----------------
```