markdown widget: allow setup of language tag

[seebi]

we should allow to setup the language here.
if a language is given, the datatype should not be written.

[frodeseverin]

Bump

[white-gecko]

This issue is also discussed on stack overflow and it is also not possible in RDF 1.1.

[white-gecko]

I've added a new feature branch to OntoWiki which implements one possible solution: https://github.com/AKSW/OntoWiki/tree/feature/multimarkdown-metadata-support