Cleanup flag parsing.

[dlorenc]

This also changes the types flag from a single comma separated list to
a repeated flag that fills an array. This changes the name from types
to type, which is a minor breaking change.

Let me know what you think.

cc @aaron-prindle @nkubala

[nkubala]

Nice, yeah I like this better than the comma separated list for sure. WDYT about having the flag be --diff-types, or -d, just for clarity? If we document it it doesn't really matter but I think --types is a bit ambiguous

[r2d4]

container-diff diff --diff-types ... ? As a separate PR it might be worthwhile to rethink the commands and flags before they grow anymore...

[dlorenc]

OK, tests passing now.