Allow handling of "custom" tokens in the Markdown widget

[davep]

With #2075 the Markdown widget allowed for easy customisation; this meant that a different parser could be used, but it also allows for adding in various plugins too. However, some plugins will result in tokens that Markdown doesn't know about.

Extend the Markdown widget so that a developer can inherit from it and add support for new tokens.

This will greatly support Textualize/frogmouth#20 and allow for something like Textualize/frogmouth#15 to be easily resolved.

[davep]

Closing as satisfied by #2803.

[github-actions]

Don't forget to star the repository!

Follow @textualizeio for Textual updates.