invalid links styling

[charlesmauch]

Currently, when invalid wikilink is detected, it's just wrapped with a <span> tag and the raw markdown is passed through which results in some ugly looking text output to the webpage.

Eg: [[link-slug|Readable Slug]] shows just as up written.

It would be nice if the invalid link could be cleaned up a little. Maybe remove the slug and the surrounding brackets, leaving the slug or label text surrounded by <span> html tags. In the above example, the result might look like <span class="invalid-wiki-link">Readable Slug</span>