Sanitize using cheerio instead of marked

Atom has a [content security policy](https://developer.mozilla.org/en-US/docs/Web/Security/CSP) enabled so sanitizing using marked is no longer needed.

This PR adds some sanitization using cheerio to strip out `script` tags and common `on*` attributes so that the console does not get spammed with CSP errors each time the markdown renders.

Closes #56
Closes #61