Create new visual tests

Refactor the component to be vertical by default, and for the "border
collapse" presentation to be provided by a modifier (via a plugin file).
These tests will indicate when the refactor provides a correct
implementation.
Refactor: vertical by default; new ws collapse

Use `font-size:0` to collapse whitespace between `ButtonGroup-item`
elements in the component. Use `font-size:1rem` to reset the font-size
in those elements to match the root font-size. (Drops IE 8 support.)

`ButtonGroup` arrangement is now vertical by default. Horizontal
arrangement provided by the `ButtonGroup--hz` modifier.

Button borders are no longer collapsed together by default.

Fix gh-3
Add "border collapse" modifier via plugin file

Provide a `ButtonGroup--borderCollapse` modifier class to collapse the
borders of buttons (assumed 1px border by default).

Fix gh-7
Update README