Syntax highlighting does not respect language specified

[nathancarter]

In the windmill theme, try writing a page containing the following code block:

```sh
git clone https://github.com/gristlabs/mkdocs-windmill.git
```

It will ignore the language specification and attempt to guess the language instead, probably arriving at some conclusion like "vim" and highlighting the s: in the URL blue (!).

For two years, mkdocs has supported disabling highlight.js, which is great because highlight.js insists on guessing the code language, ignoring the user's explicit direction, and guesses something silly, as in the example above. In all built-in mkdocs themes, you can do:

theme:
    highlightjs: false

markdown_extensions:
    - codehilite

extra_css:
    - css/code_styles.css

Please add support for this to windmill.

To see how mkdocs did it for their themes, check out, for example, this PR.

Thanks!