Add an experimental "console" CLI command.

The "console" command drops you into an IRB session in the context of the
Middleman application instance. You can look at configuration, poke around
at the sitemap and data, try out handlers (after calling self.current_path = "foo"), etc.

I'm not 100% sure how to make this super awesome, but it seems like something that could be cool. (This pull is against master, I figure it's like the metadata pages, something new and interesting).

@tdreyno, what's your take?