Add config/command for extraction

[rusackas]

It might be nice to be able to update the .pot strings (extract) and fill in the blanks with new stuff. This could be useful for CI implementation, or just updating translations on a more regular basis.

In the case of superset, we have this command to extract new strings to the .pot file.
pybabel update -i superset/translations/messages.pot -d superset/translations --ignore-obsolete

If we were to add that as config to ai18n, then we could simply run a command like ai18n extract