Add build-fixtures script
Add build-fixtures script

This will:

1. Pack all relevant Bugsnag packages
2. Install them into any fixtures in `features/fixtures`
3. Build each fixture
4. Tidy up after itself

bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js,bugsnag/bugsnag-js