Implement an option to wrap only modified paragraphs

[JulienPalard]

As msgcat now have two distinct wrapping depending on the version of gettext, powrap can no longer enforce a "common True wrapping".

Trying to enforce a common True wrapping in this condition looks wrong, but users could still have to use powrap (users of emacs po-mode that does not wrap at all need a tool to wrap, so at least me.).

But if a repo (like python-docs-fr) have no "common True wrapping" and I edit a typo on a file and I powrap the file, I may rewrap the whole file, leading to an unacceptable diff for a typo.

So maybe powrap can have an option to rewrap only paragraphs that were edited AND need rewrapping, just to minimize diffs.

See python/python-docs-fr#1825 for context.

Also while there no longer a "common True wrapping", there no longer a real need to use msgcat, we may fall back to textwrap for portability?

[JulienPalard]

I looked at polib source, and there's currently no way to not rewrap what's already correctly wrapped: polib forget the initial wrapping while parsing the file.