Allow plugins for language extension

[ebertmi]

https://github.com/jonschlinkert/remarkable

Remarkable allows to add language extensions via plugins. This would be a desirable feature.

Additionally, remarkables option would be also a idea.

[karolyi]

+1

[nikolas]

The focus of this library is to support the CommonMark spec. If you think extensions should be in commonmark you can make suggestions here: https://github.com/jgm/CommonMark/

However, I think @lu-zero has added extension support to our fork of the library, which you can get here: https://github.com/rtfd/CommonMark-py