Add (<.:>), which is like (.:) but traverses through the JSON according to the given path

This patch adds a `(<.:>)` function which is a traversal version of `(.:)`. That is, instead of taking one key, it takes a list of keys and traverses the JSON until it gets to the end.

I used it here: https://github.com/mike-burns/github/blob/master/Github/Data.hs#L358-L361

For this JSON: https://api.github.com/repos/thoughtbot/paperclip/pulls