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
> What if a line comment starts immediately after a command word?

This is not valid syntax **at the moment**. All Dockerfile instructions should be followed by at least one argument, so no line comment can occur right after a Dockerfile instruction.

> Can directives span multiple lines?

Yes. Directives can span multiple lines and I mistakenly did not take this into account. Right now I am working on it.