m.code: add a bunch of advanced .. include:: features.

m.code: add a bunch of advanced .. include:: features.

It was already useful, but this makes it even more. What's done:

-  :start-on: so it's possible to pin on something in the code without
   having to add a comment marker everywhere
-  :end-before: that can be empty, and thus simply stop at the end of
   the block
-  :strip-prefix: for removing excessive indentation as well as adding
   an ability to interpret code comments as reST.

All combined together, one can now embed Sphinx Gallery files (which can
be converted to ipynb files too) *and* still have a total flexibility
over what's done on the content formatting and layouting side, ignoring
boring portions of the source file etc etc.