Feedback: Adds a run command

Hey, I was hoping to get some feedback on this feature. I'm looking to replace the Ruby version of Foreman with node-foreman, but having a bit of difficulty since there is no run command.

Having `foreman run` would allow for things like testing tasks that would be run by a scheduler, running tests that require variables from the .env file or running something like `sails console`.

I did my best to follow the existing coding style and also tested the run command by linking my local copy of node-foreman and I was able to get `sails console` to work with my `.env` file.

Does this feature fit with the intent of node-foreman?

What additional work needs to be done before this could be merged in (error handling, tests, etc.)?

Thanks!
callback should be mandatory and should include an error if the process does not exit cleanly