Generate help infomation of task args from function docstring.

[cupen]

@task
def build(skip=False, validate=False):
    """
    Just build
    :param skip: it's blahblah...
    :param validate: it's balhblah...
    """
    pass

I can make a pr for it.

It's inspired by https://github.com/shmuelamar/cbox