Ability to add free form tags to a guide

[durden]

Currently we're categorizing guides based on a pre-defined list of 'stacks'. This is great for neatly organizing the guides in the github repository since all guides for a stack live in the same directory. However, these stacks are very generic and that poses a few problems:

1. Difficult for readers to quickly find specific content their interested in
   • For example, it's hard for a reader to see if there are guides written specifically using the twitter API. Right now a guide using the Twitter API would probably fall into the 'Interesting APIs' stack.
   • The user would have to sort through all the guides in the 'Interesting APIs' stack to find what they want.
2. Difficult for authors to tag their guides with more specific information
   • This is essentially the same as problem 1 but from the author's point of view. Author's could make it easier to find their guides if they could provide some custom metadata such as a tag.