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?