XML/HTML elements could contain Markdown

Since XML/HTML elements could be anywhere and since they can surround a
piece of Markdown then the content must be highlighted as Markdown

A more correct solution is to use a region for a non empty XML/HTML
element so to be able to check that a closing tag has a matching opening
tag but I will leave things like that because it's not possibile to
prevent spelling on match groups and so tags would be highlighted as
spelling errors

Until I find a suitable solution this should be fine
Folded syntax file, more manageable this way