Shell and magic commands

[AntoinePrv]

We should provide magic commands such a %timeit and shell execution like !ls.

Caveat: % is already used for comments in Octave. What character should we use? Or should we stick to % like Calysto/octave_kernel.

General purpose magic could be up-streamed to the Xeus project.

Idea of magic:

• https://xeus-cling.readthedocs.io/en/latest/magics.html
• https://github.com/Calysto/metakernel