Add #partial helper to Spar::Helpers

Heya, we had a need for a Rails-like render partial helper within spar, so I threw this together. I couldn't get #render working, because there was a naming collision with something in Sprockets, so I named it #partial instead. There might be a better way to do this, and I wanted to gauge your interest in having this feature in Spar at all, so I haven't written tests or documentation for this pull request. Let me know if you're amenable, and I'll go ahead and do that.