Allow extraction of docstring & comments using `pybabel extract`

[madebylydia]

Hey there. I'd like to use pybabel extract to extract my translatable strings in a .pot file, as it already does. However, I'd like to also include docstrings and comments in the extracted strings.

This is a thing that already exists in pygettext3, but not in pybabel. Would it be possible to add this functionality to pybabel?

As I pretty much need this, I could work on this addition if no one else is willing to work on this. Just LMK if this is possible to be added. Thank you.