Add a 'restart' command for restarting bots via IRC

[jlu5]

Often times this is more convenient than having to log in to the terminal where the bot is running and restart it manually.

[weedy]

while :; do ./bot --config; sleep 3; done

[progval]

Closing the issue, because @glolol has wrote a plugin to do that: https://github.com/GLolol/SupyPlugins/tree/master/Restart

Also, using the systemd unit or supybot-botchk (which I now consider the preferred ways to run Limnoria) makes it so that @quit actually restarts the bot.