Add: standard buttons

[scottaohara]

Largely straight forward, but there are a few gotchas to consider when styling

For instance:

• buttons with forced multi line text (aka using display block elements inside buttons).
• ensuring buttons have noticeable contrasts shifts, or other visual indicators between states.
• buttons with nested spans/svgs that can get in the way of click/touch events.
• document issues with display
• document all of this (and whatever else is thought up)