lint on relative links to md files using html extension

[gnzlbg]

mdbook should lint against relative links to markdown files that use the html or other extensions. For example, linking against

[foo]: ../bar.md#foo

using

[foo]: ../bar.html#foo

should warn or error that the md extension should be preferred instead. Otherwise if the markdown is compiled to something else, like pdf or some other format, those links will break.