Add an animation to comfort the user

Sometimes msfconsole takes a little while to start. See #3725

This adds a fairly common ASCII spinner to the startup sequence, mainly to comfort the user that their machine isn't locked and things actually are happening while they're waiting to get hackin'.

I haven't spec'ed it, and the code organization isn't great, and there may be problems with printing to `$stdout` in the first place, so consider this PR more of a cry for help than something immediately landable.

That said, it works for me.
See https://github.com/rapid7/metasploit-framework/pull/4014#discussion_r19033345. :)