Add command to show available snippets

It's something I've always missed in atom. I've implemented it as [standalone-package](https://github.com/frk1705/atom-available-snippets), but I really think this is something that belongs into the main package!

![example](https://cloud.githubusercontent.com/assets/5219415/3142144/fd9eb42e-e9ac-11e3-9599-0a22e95d4980.gif)

~~I have not yet figured out how to use snippets.coffee with a require to replace my `getSnippets()`-method and the calls to insert a snippet using~~
 ``` coffee 
snippets: -> atom.packages.activePackages.snippets.mainModule
[...]
@snippets()?.insert item.snippet.bodyText
```
~~but I think you might know how?~~