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!
A callback should be used here so that `nf` can exit with the same exit code as the child process.