Literate programming

Ability to show value of kernel variable in MD cell.

Do not view example with nbviewer, it does not support it yet. 

allow to write code like
```
>>> a=1
```
then in MD cell `a=::a::` which will be rendered as `a=1`

This does not allow repr_tex, or anything else as it uses `user_variable`.
Should we consider extending protocol ? Did I missed a clean way to do it ?