Support for PowerShell syntax.

This work is based largely upon the work of @nblumhardt and @Lavinsky in [OctopusDeploy](https://github.com/OctopusDeploy/Library/blob/master/app/shared/presentation/highlighting/powershell.js), which was found via the [mailing list](https://groups.google.com/forum/#!msg/highlightjs/Gt7xPXoTWQM/jTc9lmiCOWoJ).

I'm pretty sure it still isn't conclusive, but it is a good start.

Also, I think it makes sense to recognize the bulk of the work from the group at OctopusDeploy - should I just add them as contributors, too?

Hope this helps!