Template descriptions

[eriknstevenson]

Hi! This is my first stab at #1571. There didn't seem to be much of a consensus on how best to implement this feature so I went off of @mgsloan's suggestion

An alternative implementation not blocked on #1595 would be to have a single file in the templates repo that associated template names with descriptions / other metadata.

Running stack templates now downloads and can display optional metadata from a .yaml file located in the stack-templates repository.

The file is setup like this:

quickcheck-test-framework:
  description: a library for random testing of program properties
servant:
  description: a set of packages for declaring web APIs at the type-level
ghcjs:
  description: Haskell to JavaScript compiler, based on GHC

Currently the data type I implemented supports description and author (not used for anything at the moment) fields, however this could be extended relatively easily.

I'm a newbie so criticism is definitely welcome and appreciated.

Thanks

[eriknstevenson]

I'm going to clean this up and get it compiling with ghc-7.8.4.