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
Could we use `npm version` to update the version and form a commit?

Like:  https://github.com/alphagov/govuk_frontend_toolkit_npm/commit/ea82015954e30242b3d023c5ddbc6238310abc6a#diff-04c6e90faac2675aa89e2176d2eec7d8L36