Summary:
Adds a Markdown component to the component library.
The react-markdown library doesn't work out of the box because elements it outputs don't have any styles, e.g. h1, h2, p, em, etc.
So I've added a custom renderer to give styles to each of these that workplace markdown uses.
The only one I haven't done at the moment that I'm aware of is numbered-lists.
There's probably a way to get back the original styling of these elements so we don't need a custom renderer, but this works for now, and also allows us to customise their appearance.
Reviewed By: passy
Differential Revision: D18533085
fbshipit-source-id: a4332a11d34d577a6300074e4dde126362d46a6b
352d9c82cb