Escape jupyter magic commands

[mwouts]

Python cells in R markdown files and plain python files cannot contain jupyter magic commands. These should be escaped, in a revertible way so that reconstructed jupyter notebook remains unchanged.

[mwouts]

Not all lines that start with % are jupyter magics... Pytest features, and more generally python decorators also start with the percent sign. I will restrict the escaping of magic commands to the built-in magic commands