please add subcommand to determine lock state

I find that I'm wondering, "is my git-crypt repository locked, or unlocked?"
Sometimes, I cannot remember. Then have to go poking in files to see...
unless there is some easier way I missed?

Suggest a command, perhaps `git-crypt lockstate` that tells the user
whether it's locked. Give this a 'quiet' flag to have no output (maybe
in this case, name it `git-crypt islocked` and have zero be 'yes').

A [maybe better] alternative would be adding this information to
`status` output (although `status` is more parsable as-is, one file per line)
or giving it an option (maybe `-l` or `-k`) to determine the locked state
(which could then be combined with quiet flag from #68 for no output).