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
This state looks like it mostly works, but does so by accident. Is the rest of the line after a command always the continuation of the command? What if a line comment starts immediately after a command word? You can include a rule without regex (always matches) that has `next: "start"` to also go back to start when nothing else matches, but then you might as well make your directive rule immediately consume the text after it (using groups and an array of `token` styles).

Can directives span multiple lines?