Generating docs does not work

[trevorcampbell]

If I run

python3 tasks.py docs

from the repository root, I get the following output:

Traceback (most recent call last):
  File "/home/trevor/repos/dsci/nbgrader/tasks.py", line 176, in <module>
    ns.func(ns, args)
TypeError: docs() takes 1 positional argument but 2 were given

This appears to be related to this recent commit:

https://github.com/jupyter/nbgrader/blame/af47e5eaf880d456f79030e68c0feb0de1fb6313/tasks.py#L176

where these two functions were not updated:
https://github.com/jupyter/nbgrader/blame/af47e5eaf880d456f79030e68c0feb0de1fb6313/tasks.py#L35
and
https://github.com/jupyter/nbgrader/blame/af47e5eaf880d456f79030e68c0feb0de1fb6313/tasks.py#L46

It should be fairly straightforward to fix these by adding the args argument to docs() and cleandocs()