Rewrite readme for unified, more examples

* Lowercase filenames to scream less
* Add `remark-cli`, `remark-preset-wooorm` to format prose
* Use imports and fancy new JS features
* Add link to a markdown cheatsheet, as many questions are about that
* Add details tags showing equivalent JSX for examples, to clarify how markdown
  works
* Refactor options list, clean signatures
* Add examples on plugins, plugins w/ options, renderers (syntax highlighting),
  and combining plugins w/ renderers (math)
* Add sections on why to use this, security, contributing, related projects
* Add test for `react-katex` integration

Closes GH-70.
Closes GH-191.
Related to GH-474.
Closes GH-479.
Closes GH-480.
Related to GH-483.
Closes GH-485.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Espen Hovlandsdal <espen@hovlandsdal.com>