New command line option '--user-dir' (#440)

Summary:
Following discussion on official fairseq (https://github.com/pytorch/fairseq/issues/438), I added the `--user-dir` option to the command line. The user can now specify a path in order to import a custom module with proprietary tasks, architectures and so on.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/440

Differential Revision: D13651721

Pulled By: myleott

fbshipit-source-id: 38b87454487f1ffa5eaf19c4bcefa0b3b15a8f43