Allow passing tags as part of the task

`_stub_task` currently accepts tags separately from `**kw`. It would be nice to be able to pass tags as part of `**kw` (for example, when calling `task_add()` in Bugwarrior.

In the future it would probably make sense to refactor `_stub_task()` so that it only accepts one parameter, `**kw`.

What do you think?