Adding in show missing strings command

This command allows someone to validate that all the strings
in their code base are translated/have corresponding entries in their
respective json file. It will also validate in the opposite direction.

[#72313196]
Refactor to the MAX.
Handles case where first argument is another function.
Only parse .go files.
Adds file name to missing strings output
Merge pull request #18 from crhino/master

Adding show-missing-strings command.

Just wanted to have this be a separate pull request to track the work a bit easier. Hope you don't mind me merging this in @maximilien!