Implement a MarkdownProvider

[johno]

This will add a context that provides an element to component mapping, and any other globally available components.

As an example API, this will render Heading for the h1 element in readme.md.

import React from 'react'
import { MarkdownProvider } from '@compositor/markdown'

import { Heading } from './ui'

import Readme from './readme.md

export default () =>
  <MarkdownProvider components={{ h1: Heading }}>
    <Readme />
  </MarkdownProvider>