Nuemark extension system

### Is your feature request or improvement related to a problem? I would like to compile RDF-a HTML from my markdown. So I can write: `my name is (name)[john doe]` and it compile to `

my name is john doe` But I think this problem can be generalized to anyone wanting a special markdown syntax. ### Solution you'd like For example, Python's markdown library is extensible with a [RDF-a module](https://github.com/tetherless-world/markdown-rdfa) which make the lib compile a special RDF-a syntax. I think it's nice to be able to hack the compiler with custom regex ( the extenssion is only 100 lines of code long ) and let users the choice to opt-in for such features. ### Alternatives you've considered - Pre-compile with python markdown - use Nuemark custom tag ### Additional context