<tt> tags are not skipped by EmojiFilter

The rest2html command in github/markup outputs HTML4, so it wraps rst inline literals with `<tt>` instead of `<code>`. The emoji filter doesn't recognize `<tt>` as a tag that contains literal content, and emoji are substituted where they shouldn't be. 

Toy example: https://github.com/moskvax/test/tree/master/test
Real example: https://github.com/mpv-player/mpv/blob/master/DOCS/man/vf.rst