CLI should gracefully shut down (clear js-ipfs lock)

[Schwartz10]

When ceramic-cli gets shut down, the js-ipfs repo lock isn't properly released. Users have to wait at least 10 seconds for that lock to get released before spinning the daemon back up. It would be great if ceramic-CLI could gracefully shut down to avoid this, which would pave the way for other graceful shutdown actions that might be useful for future use cases

[stbrody]

This probably just needs us to regular a signal handler and trigger graceful shutdown (call ceramic.close()) when we receive a SIGTERM or other expected signals