Lint example references by name

[peterbe]

If you look at https://raw.githubusercontent.com/mdn/stumptown-content/master/content/html/reference/elements/input/input.md there's this:

examples:
    - examples/examples_overview
    - examples/Styling_input_elements
    - examples/Custom_error_messages

But one of those don't exist. There is examples/examples_overview. There's a examples/Examples_overview though. Easy mistake to make. Let's prevent it from happening again.

[peterbe]

Also, on macOS and Windows, I don't think it notices because I can do this:

> var fs=require('fs')
undefined
> fs.readFileSync('pAcKaGe.JsOn', 'utf-8').length
2147

So these kinds of problems would be hard to catch on a case INsensitive system.

It would be nice to not have to rely on a linter but catch these kinds of problems early and "more natively". I.e. we open a lot of files and stuff, primarily based on clues from front-matter and wherever we do that it would be nice to get these things figured out when it's needed and relevant.