Customizable LaunchDaemon via a command-line flag

This is in response to #411. Allowing you to specify arguments on the
command-line has more edge-cases than I'd prefer, so I think this is a
bit more of a sustainable solution, especially given that you're already
supplying the tool with a path to your config file (now you can just
track one additional file).
Merge pull request #436 from facebook/launchd-customization

Customizable LaunchDaemon via a command-line flag