add support for xmpp: uri in markdown syntax

[goffi-contrib]

When adding and XMPP uri in the following form:
[[email protected]](xmpp:[email protected]?join)
the uri syntax is shown raw instead of linking to the room as expected.

[Changaco]

Okay, so here's the Markdown situation:

We're currently using Misaka 1.0.2, an obsolete version based on the abandoned Sundown lib.

We have two possibilities:

• upgrade Misaka to 2.0.0
• switch to Mistune

According to this benchmark Mistune is around 20 times slower than Misaka, but it's pure python so there are no dependency issues, and it allows customizing the lexer in addition to the renderer (we don't need that feature right now though).

In both cases we need a Renderer subclass to sanitize links. We've been using the HTML_SAFELINK flag of Misaka 1, but Misaka 2 and Mistune don't have this feature built-in, and anyway we need to have sufficient control over the filter to allow XMPP links and close this issue.

[Changaco]

I tried upgrading Misaka first and it worked so I went with that. See #502.

[goffi-contrib]

That was quick, thanks :)