add an option to only run the subcommand when files change

[mmerickel]

Right now hupper runs the command immediately and then kills / restarts it when files change. Some interesting use-cases open up if it has an option to only run the command when files change. For example:

hupper --wait-for-changes -- docker restart app

If the command exits then hupper already will wait for files to change and run it again. I think this issue is just about not running it at startup.

This is likely contingent on changes in #19