Add support for on/off comments

[pawamoy]

Closes #193

[pawamoy]

Not sure yet how you'd like to add support for rST and Latex.

rST comments would look like this:

..
    blacken-docs:off

.. code-block:: python

    f(1,2,3)

..
    blacken-docs:on

And Latex ones like this:

% blacken-docs:off

\begin{minted}{python}
f(1,2,3)
\end{minted}

% blacken-docs:on

Should we extend the comment regex to match all three kinds of comments regardless of the source file type?

[pawamoy]

OK we now have a single regex for the three syntax, and tests for each.

[pawamoy]

Pre-commit doesn't use the modified code, therefore my examples in the README got formatted 😅
I suppose we should first release, then modify back to remove spaces in f(1, 2, 3), and this time it won't get formatted.

[pawamoy]

Also note sure how to cover line 120.

[pawamoy]

@adamchainz gentle ping 🙂

[pawamoy]

@adamchainz is it OK if I ping you from time to time? That's what I ask from others when I don't answer, but you might work differently, and I don't want to bother you 🙂

[adamchainz]

Please don’t ping

[pawamoy]

I stopped using blacken-docs some time ago, so I'll close this PR, but feel free to reopen it if you or other users are still interested in getting it merged 🙂

[adamchainz]

I still intend to add this

[adamchainz]

Thank you for the PR, I finally found time to review and finish it off. You may be interested in reading the intermediate commits.

[pawamoy]

Awesome, thanks. You made some clever optimizations and simplifications that I will keep in mind!