Add Mallard and Ducktype mime types

I open this issue for adding Mallard (XML) and its sibling Ducktype (lightweight markup) to the shared-mime-info database.

I think the mime types for those belong more to shared-mime-info than shipped as part of a specific application. Mallard is supported by several applications: text editors such as gedit and Vim (syntax highlighting), the Yelp documentation viewer/browser, command line tools, and probably others.

I see that there is already #64 (closed) with attachments on the corresponding bugzilla ticket. This can serve as a basis.

I will send an email to the Mallard mailing list with a link to this issue, so that upstream developer(s) could give advice or could review a potential patch.