Auto release new versions of the toolkit

Update the submodule and use the version file from within the
toolkit to get a new version. Use that new version number to publish a
new version to npm. Commit the new version and updated submodule.

This is based on the work from https://github.com/alphagov/govuk_frontend_toolkit_gem/pull/8
We could, that would create two commits (one bumping the submodule and one bumping the version). Not sure if that is better or worse.