Add OcticonButton component

[shawnbot]

The OcticonButton component renders a <button> with an <Octicon> in it, and adds the requisite Primer classes to make it not look too button-y. Here's an example of a close button (ref: #114):

import {OcticonButton} from 'primer-react'
import {X} from '@github/octicons-react'

export default function CloseButton(props) {
  return <OcticonButton icon={X} {...props} />
}

A couple of other things to note:

• Octicon buttons use the text-inherit class to pass the inherited color CSS property to the Octicon SVG element.
• They pass disabled and onClick to the <button>, and support all of our margin and padding whitespace props (m, p, mx, py, etc.) so that it's easy to space them out and increase their hit area.
• They pass the icon and size props to the Octicon component.
• The label prop is rendered as the aria-label attribute.

[emplums]

Looks great!