"At" symbol, i.e. @, should be allowed in URL

[rgaiacs]

Description

Some links might have the "at" symbol (@). For example:

• https://hackmd.io/@sgibson91/hubs-team-meeting
• mailto:john@doe.com

mystmd parses the "at" symbol (@) when it is inside a URL as a BibTeX reference ID.

Proposed solution

Delay the parse of the "at" symbol (@) to after the parse of URLs.