Add recursive usage option

[afonasev]

Need for:

add_trailing_comma.py -r ./source ./tests

[asottile]

Since this is nearly impossible to get correct and very complicated I'd suggest instead running through pre-commit -- pre-commit is going to do a better job at running linters than any linter ever could and has inclusion / exclusion / etc. out of the box.

Here's a similar comment from another project I maintain: asottile/pyupgrade#46 (comment)

[afonasev]

Ok, thx