Introduce Dockerfile mode

This pull request introduces syntax highlighting for [Dockerfiles](https://docs.docker.com/reference/builder/#format) by utilizing the [simple mode add-on](codemirror.net/demo/simplemode.html).

### Features
- Instruction highlighting
- Line comment highlighting
- Inline comment highlighting
You probably want to add `"("` and `")\b"` here so that `addiction` and such (words that start with a directive keyword) don't match.