Adds possibility to build extension files (Chrome and Safari) from
command line.
To run from the project directory:
python tools/build.py [meta]
If the optional `meta` argument is set, then only the manifest and
language files are uptated.
Without that everything is being built (extension files too) into the
`dist/build/version_number` folder.
For Chrome there will be two files, a crx, and a .zip file which
includes the key.pem private key (so this must not be shared,
it's just a bit help for publishing it to the Chrome Web Store).
Beside the extension files, update-files are generated too (for self
hosting - Safari needs it).
