Add an option to preserve translated and extracted comments

[liborjelinek]

I often add translator and extracted comments to PO files. Unfortunately, sphinx-intl update deletes these valuable comments.

It would be super-useful to have an commandline option to keep them. Or better, IMHO comments should be preserved by default, i.e. have an option to delete them.

[tk0miya]

+1: It must be useful. Do you have an idea to implement it? Contributions are always welcome :-)

[silopolis]

Is that really a Sphinx issue or a Babel one ?