add collection directory location

Currently, a Collection's "relative_directory" is set to the Collection label. The [documentation](http://jekyllrb.com/docs/collections/#using-collections) is a bit confusing here, because one can apparently access the relative_directory, but not configure it.

This pull request gives users the ability to specify a directory location for their Collection. Rather than have the directory be limited to a collections label, I believe it will be better for users, including content creators, to optionally organize their collections into a folder or folders. 

I'm adding this at an early state to open discussion to make sure I'm on the right track (code-wise and idea-wise). Maybe I'm the only person who obsesses over clean directory structures and maybe there's more comprehensive approach than what I've taken here.

**Use case**
```
src/
- _documents
     _authors  
     _books   
     _translators  
     _editors  
- _data  
- _includes  
- _layouts  
```

Still being inexperienced with Ruby, I've listed out what I think I need to do to accomplish this if we determine it's the best coarse of action. Comments appreciated.

- [ ] decide on proper term (currently, I have "collection_location")
- [ ] define configuration variable
- [ ] set it so the location is optional
- [ ] set the default to source
- [ ] make the location available to Liquid
- [ ] add tests
- [ ] mention in documentation (right now there's nothing set for configuring collections)