Cache comments

* Added Lager for logging
* Clean up unused variables
Cache rendered html from markdown

* Separate the rendering of gist content and comments.
* Each rendered html is cached with the key is the markdown itself.
* It's kinda hacky to take advantage of blockquote markdown style to
differentiate between comments, but we don't want to call render for
individual comment.
* Change the order of arguments in `GistsIO.Cache` to take advantage
of pipe in Elixir.