Convert to click for CLI options

click is a pretty cool library for handling command line options. It's fairly popular and has some properties that make it nicer for Mailman than our homegrown approach. There are a few downsides, but those might be related to my current knowledge of the library.

Adopting click may make sense if it means less code for us and a better plugin architecture for subcommands.

Close #319 (closed) and #346 (closed)