Refactor: more robust layout; better defaults

* Vertical alignment by default.
* Use `font-size:0` / `font-size:1rem` to squash whitespace (drops IE 8 support).
* Move "border collapse" presentation into a plugin.
* Better scoping using `>`.

(This branch assumes that the buttons will be full width. See suitcss/button#7)

Review request @suitcss/contributors