Missing email styles

[garrettbryan]

I fixed a bug with in a different project called "Gitalk".
gitalk/gitalk#224

Turns out that there are infrequently used Github classes for email comments.
Would you accept a pull request?

[sindresorhus]

The CSS is generated, so needs to be fixed here: https://github.com/sindresorhus/generate-github-markdown-css