feature/as-a-user-i-should-be-able-to-use-markdown-in-my-document

This is Ready.

Branch resume:

It includes a custom markdown (the one that includes StackOverflow) editor for converting markdown to sanitized HTML in real time.
It allows to type markdown and to see it interpreted as HTML in realtime, when:
Creating a new document
Editing a document
Seeing a document
Has the corresponding specs.

@TheNaoX can you take a look please?