Add drawer component.

@tgenoni I added a drawer component. It is not done – this is an incremental commit for early feedback. Main goal of this is a prototyping playground for me to try out some designs/behaviors; however, I expect when it's done that this will be a reusable component in the actual Optimizely repo. But any high level feedback on how I'm structuring and doing things so far is welcome. I won't merge until it's actually complete (ish).

I'm also aware that I'm doing my Vue stuff differently from how Jordan and others are doing it in production code, but I will fix that later (maybe).