Design for interface to allow adding in and building your own version of _s

This is taking the conversation over here https://github.com/Automattic/_s/issues/659 to underscores.me and hopefully kicks off the things discussed. I thought I'd start the ball rolling with a speed mockup session. This is just a short rough idea but hopefully means we can have a starting point.


A few points:
- There are 2 methods of display and I've done that because as things grow perhaps grouping should happen.
- I didn't have the font so that needs to be added in to fit. I went with default Arial to ease.
- I've kept the branding, simply adding a block if you want to customise.
- Wording is just there for now, we can decide on that but it's to demonstrate.

Start by reviewing the discussion in Automattic/_s issue 659 and the linked underscores.me mockup. The issue needs an agreed interface scope, wording, typography, display modes, and customization approach before completion can be judged.

Written by the indexing model from the issue text.