Add List.keysort

Receives a list of tuples and sorts the items at position `pos` of the tuples
Fix a bug on OptionParser where missing options appeared on the final result
Improve docs for Mix
reverse @__behaviour_docs__

These are top accumlated as module attributes, so are therefore stored
in reverse order to how they were added. When we get them back out,
reverse them again so they're back in the right order.

This is used by ex_doc, and callbacks are listed backwards otherwise
Merge pull request #1461 from madlep/fix_backwards_behaviour_docs

reverse @__behaviour_docs__
Merge pull request #1458 from guilleiguaran/list_keysort

Add List.keysort
Reverse behaviour docs at compilation time
Update CHANGELOG