Implement --init-missing option for pybabel update

[RuRo]

Currently, if an output .po file doesn't exist during pybabel update, the update fails when trying to open the missing file. This PR adds a boolean --init-missing option. If this option is enabled during an update and an output .po file is missing, it is created and initialized the same way it would be by pybabel init.

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Before this PR you had to run two commands like

pybabel init   --domain=${DOM} --locale=${LOC} --input-file=pot/${DOM}.pot --output-dir=po/
pybabel update --domain=${DOM} --locale=${LOC} --input-file=pot/${DOM}.pot --output-dir=po/

if po/${LOC}/LC_MESSAGES/${DOM}.po didn't exist.

If the file did already exist, you had to only run the second command (otherwise, running init on an existing file overwrites it, removing any previous translations, which is not desirable).

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After this PR you will be able to run a single command and get the expected behaviour, no matter if the output file existed previously or not:

pybabel update --init-missing --domain=${DOM} --locale=${LOC} --input-file=pot/${DOM}.pot --output-dir=po/

[RuRo]

@akx Sorry to ping you, but what's a cowboy gotta do 'round here to get his PR reviewed 🤠?

[akx]

The code looks good, thank you.

I'd appreciate a non-trivial test that does test that this option also does initialize the files as it promises.

[RuRo]

@akx Done.

[akx]

Looks good, thanks. Sorry for the delay in reviewing!