translations: refactor cli

[zzacharo]

Workflow to translate

 1) Add a new language
    npm run init_catalog lang <lang>
 2) Extract translation keys/values
    $ npm run extract_messages
 3) Update the src/lib/translations/messages/index.js file
    import TRANSLATE_<lang> from './<lang>/translations.json'
    export const translations = {
      ...rest,
      <lang>: { translation: TRANSLATE_<lang> }
    }
 4) Install the transifex-client
    $ pip install transifex-client
 5) Push source (.pot) and translations (.po) to Transifex
    $ tx push -s -t
 6) Pull translations for a single language from Transifex
    $ tx pull -l <lang>
 7) Pull translations for all languages from Transifex
    $ tx pull -a
 8) Compile .po files for all languages
    $ npm run compile_catalog
 9) Convert .po file for a single language
    $ npm run compile_catalog lang <lang>

Added as bonus some greek translations :)

[mb-wali]

so far so good!

[zzacharo]

Once merged update the #319 with the new workflow :)

[mb-wali]

LGTM!