Using Vorpal without `show`

[ccorcos]

There are a few quirks I've been noticing when using Vorpal without .show().

• without process.on('SIGINT', () => process.exit(2)), it doesn't always properly exit and I'm left in some weird limbo where ctrl-c will just print ^C. Even when I kill the process, ctrl-c still doesnt do anything but print. its bizarre.

• if you run a command with incorrect parameters, you're left inside the Vorpal shell rather than being dumped back into the commandline

• the delimiter is printed out once at the beginning and once at the end even when you never call .show()

[ccorcos]

That said, I'm using a Vorpal command to run an express server and I'd like to be able to listen for a kill signal, close the server, and return a promise to do other cleanup. This whole SIGINT thing is cuasing problems... :/

[minhchu]

hope your PR works this solution used to work but not the lastest version of vorpal