Added support for TeamCity progress messages

Fixes #242

To be perfectly honest, I kinda hacked this in to get it to work for our use case. Please let me know if there are changes you'd like me to make so that it fits better into the Pester architecture! If you're not able to take the PR now, that's OK. It would be helpful to have a quick heads-up if you don't think you'll be able to look at this for a while so we can set up our own temporary fork of the code.

It would be :sparkles: super-awesome :sparkles: if you could create a new release when/if you integrate this PR, but not essential :)
Oh man, I didn't even realize this was still here. Talk about hacks :). I see what you meant, I'll totally move this in to Describe.ps1!