remove command: removes what generate produces

I basically wanted the opposite of generate. Is this something you might want? 

It doesn't explicity try to stop any service, just like generate doesn't try to start a service.

So one could run:
```
ndm generate # produces files (e.g. in ~/Library/Launchagents)
ndm start
ndm stop
ndm remove # removes files (e.g. in ~/Library/Launchagents)
```
And everything initially generated by ndm would have been removed.


If you want I could change it to be like start/stop/restart where it can operate on a single service or all services.