Feature: expand context

[RudolfMan]

The ability to expand the context of the code around the change.

There are definitely stuff to improve.. especially in my frontend spaghetti JS (not sure if it makes sense to rewrite Diffs views to live_view since that page isn't meant to be live)

So currently onclick on any button "expanUp" or "expandDown" script tries to read numbers from lines before and after the button and makes the request:
GET /diff/:package/:version/expand/:from_line/:to_line/:right_line?file_name=<file_name>

In the backend we unpack that file from, extract the requested chunk and render the html

To be able to unpack particular file I propose a patch to hex_core to add :hex_tarball.unpack/3 to accept the list of files. see hexpm/hex_core#95