Add remark-gfm for Github flavored markdown support

[amcclain]

It would be great if the markdown component we ship with Hoist supported GFM, in particular for table support.

We should add the remark-gfm package as a dependency and install it as a plugin by default. We can review and determine if we want to expose any props to customize. While at it, we should update to the latest major release of the core react-markdown library (v9)