Subdirs

This pull request I expect to be more controversial than my configuration one.

This patch allows for bundles to target subdirectories of the cloned repository, or files directly.  I believe this is a good and important feature, as it will allow users to target plugins with non-standard structure or naming.  From reading your conversations with the author of Antigen, I take it that you feel strongly about enforcing a standard (.plugin.zsh), but not all developers will follow that.  I think forcing users to fork every non-standard repository they want to use is a little extreme, and allowing users to specify a subdirectory or file within the main repository will give the freedom to use these plugins without having to maintain forks.

My haskell experience is... limited.  I haven't used it for some time, and I never used it much in general, so I just did my best to make the smallest changes to make this work.  I'll understand if you find problems with the code itself, and if you do I would appreciate any suggestions you may have.

If you disagree with this patch's functionality, I'll understand, but just like you more speed and simplicity than antigen offered... I need the speed and simplicity with some flexibility.  But I'm not keen on maintaining a fork, especially in a language I'm not proficient in (though I mean to learn it well when I have more time for that), so if you are willing to bend on this philisophical point, I would love to get this functionality in.