[REDO] Add system props

[shawnbot]

⚠️ This was originally done in #155 for v0.0.12-beta, but we decided to revert it and introduce the API changes in the next release, pending much-needed docs.

This introduces something approaching a "system" for shared props that are intended generate classes matching those generated in #147.

The mappers:

• borderColor, e.g. borderColor='red' renders class='border-red'
• borderRadius e.g. borderRadius={1} renders class='round-1', responsive
• color, e.g. color='blue.4' renders class='color-blue-4'
• display does display='flex' → class='d-flex' and is responsive
• flex implements all of the FlexContainer mappings (maybe it should be called flexContainer?)
• fontSize does fontSize={0} → class='f-0' (note the hyphen for consistency), responsive
• margin, padding, and the composure of the two in spacing; all responsive w/edge variants
• position, e.g. position='absolute' renders class='position-absolute', responsive

Here's how they're used right now:

import {composeWithPropTypes} from './props'
import {bg, color} from './mappers'

const mapProps = composeWithPropTypes(bg, color)

export default function MyComponent(props) {
  const {className, children, ...rest} = mapProps(props)
  return <div className={className}>{children}</div>
}

MyComponent.propTypes = {
  ...mapProps.propTypes
}

Also in this PR:

• New jest test matchers that make it easier to test classname output
• Inline styles from system.css thanks to the magic of preval
• A new "Props" tab in the x0 docs that demos color, fontSize, margin, and padding props