Code does not have caption

[artragis]

We need a plugin to embed code caption.

I think this plugin must :

• embed the "caption" attribute in the MDAST code element
• add a "codecaption" element

The main problem will come when we will translate AST to latex as we need to retrieve the legend inside the tree.

Example of code:

    ```python
    bla('bla')
    ```
    Code: Bla legend