Added foxx flag for ArangoDB Foxx globals

[ArangoDB](https://www.arangodb.org/) is a document and graph database that allows defining APIs in (mostly) node-compatible JavaScript apps called [Foxx apps](https://www.arangodb.org/arangodb-foxx).

This PR adds a `foxx` flag analogous to the existing `couch` flag for Couch apps. As there were no tests for `couch`, `phantom` and `shelljs` I haven't added any new tests.