format for restructured text (rst)

[keewis]

This introduces formatting support for code in rst blocks.

There are still some issues:

• the first indentation level within code blocks is overwritten with four spaces
• ~blackdoc doesn't search for *.rst files~
• ipython syntax (starting a line with !, @, %): works, but might be fragile
• ~code in other languages not ignored~
• ~line decorators:~

  @verbatim
  In [15]: ...

[keewis]

this looks good enough for a merge to me, but I'll wait a few days to re-evaluate. Also, I'm skipping cell decorators (since that's kind of difficult to parse) and the indentation level issue (but maybe we could make the depth configurable).

Some experiments for free time: replace the code for extracting ipython prompt code with the ipython module (or whatever the ipython sphinx extension uses).

[keewis]

the cell decorator works, so I guess everything else can be left for new PRs