Make tools/generate-test-fixture.js take a -m/--module option for `sourceType="module"` mode

Currently the `tools/generate-test-fixture.js` utility is incapable of parsing module-only syntax (like import/export).
This adds a `--module` or `-m` flag to that script to put esprima into `sourceType="module"` mode when parsing

https://code.google.com/p/esprima/issues/detail?id=614