Allow user to opt into console output with asynchronous runs

Thank you for the Grunt plug-in. I've found it very useful.

I think a *relatively* common use case for the existing `async` option is for developers who want to start the local server followed by a `watch`, and then later kill both with `Ctrl + C`.

For me, a new option to opt into sending the server output to the console rather than `/dev/null` would be immensely useful. Having both `watch` and server output go to the console together might seem like it would be a mess, but it's completely usable since many people alternate between saving a file (triggering `watch` output) and reloading a web page (triggering server output).

The existing "unable to determine success of" message might not be entirely appropriate when `asyncOutput` is enabled, but I wasn't sure, so I left it alone for right now.