CLI command(s) to add a new language

[rusackas]

On Superset, we can create a new language with pybabel init -i superset/translations/messages.pot -d superset/translations -l fi to create Finish, for example. If this command were configured in ai18n, we could use the AI18N_TARGET_LANGUAGES config to create missing .po files, or add a new command, e.g, ai18n lang fi to create the new .po file (and... maybe... fill it in)