Fix error in lang attribute parsing

[borgar]

The textile %["red":https://example.com].% is incorrectly parsed.

The resulting HTML is:

<span lang="&quot;red&quot;:https://example.com">.</span><

But should be:

<span><a href="https://example.com">red</a>.</span>

Likely it is best to restrict what input is allowed in a lang attribute.