Add silent option for startup

[strokirk]

Hi,

Currently the rundramatiq management prints each tasks module it finds on startup:

* Discovered tasks module foo.tasks
* Discovered tasks module bar.tasks

This output can be quite long if you have many tasks modules.

It would be nice if this was possible to silence by adding a -v0 option to the command.

Relevant code:
https://github.com/Bogdanp/django_dramatiq/blob/master/django_dramatiq/management/commands/rundramatiq.py#L176