Add @collection directive

[edalzell]

Implements statamic/ideas#212

[jasonvarga]

This is a nice start 🎉 but what about parameters?

[edalzell]

What about ‘em?

[jasonvarga]

As it stands you can just get all the entries. What about limiting, filtering, pagination?

[edalzell]

Oh you want FAAAANCY stuff.

[edalzell]

@jasonvarga any ideas how you parse parameters passed into custom directives? They come in as a string.

[jasonvarga]

No but I imagine you can look at some of the native Laravel Blade directives, like @include that allow arguments.

[edalzell]

done @jasonvarga. Didn't do pagination as it returns something different and I didn't want to deal with it now.

[jasonvarga]

This is great Erin, thank you for the PR. However I don't think this is quite what we'd want in the core in it's current state. You should be able to plop all of this in an addon very easily though.