Add utils.parse_unified_diff function

This will read stdin using utils.stdin_get_value and parse it into
a dictionary similar to pep8/pycodestyle's parse_udiff function.

This differs from that function in the order in which it parses the
diff and also in that it does not try to filter the paths in the diff.
It lets a different level take care of that for it.

This will likely be used to send paths to the FileCheckerManager and
also send the ranges to the StyleGuide so it can filter errors not in
those ranges.