Better management of multiple stacks

[awrichar]

Would be nice to:

• list which stack is running (if any)
• get a friendly error if you attempt to start a stack when another one is already running

Could also discuss if running multiple stacks simultaneously should be supported - right now every stack picks the same port numbers so they will always conflict.

[tobigiwa]

I have a pull request on the first one, which introduces a ps command. The second would definitely throw an error and it possible to run more than one stack simutaneously now if the port was specified at init