Make the "git appraise show" command accept the same diff flags as the "git show" command.

In particular, "git show" supports -p/-u/--patch flags for generating the diff (patch).

If we go with the -p/-u/--patch flag, then we should also add a -s/--no-patch and make the default configurable. I'd also want to keep the --diff option so as to not break any scripts that have been written against the tool.

Another thing to consider is whether or not diff should be promoted to its own subcommand (e.g. "git appraise diff"). If we do that, then we should try to make it as consistent as possible with "git diff".

This came from the suggestion @andreineculau made in #39