Make dramatiq set up the import path automatically

Per https://github.com/Bogdanp/django_dramatiq/issues/5 modifying `PYTHONPATH` can cause unexpected issues w/ module loading due to the fact that `PYTHONPATH` gets prepended to `sys.path`. Dramatiq should expose a `--path` CLI flag that defaults to `.` and **append** that to `sys.path` instead of requiring users to fiddle with `PYTHONPATH`.