ButtonGroup

[mkernohanbc]

New component idea (see @ty2k comment in #437 for context):

ButtonGroup is a utility component similar to Form, intended to help users easily create more complex UI compositions.

Its purpose is to provide a simple styled container into which the user can pass multiple button components, to be rendered horizontally alongside one another.

ButtonGroup will be useful as a subcomponent for:

• Header
• Form
• Inline Alert
• Future content components like Card and Callout