Allow invoke to run tasks with type hinting

[rsxm]

Issue #248 mentions type hinting, but I think that is for Invoke to use type hinting internally. This issue is to allow Invoke to run tasks with type hinting annotations.

With current master, when defining a task like this:

@task
def hello(name: str = 'Anonymous'):
    print('Hello, {name}!'.format(name=name))

I get the following error message when trying to run this task:

ValueError: Function has keyword-only arguments or annotations, use getfullargspec() API which can support them

I followed the advice and replaced getargspec() in tasks.py with getfullargspec() and everything seems to work.

I don''t know the full implications of this change, but I'll create a fork and run the tests and see if something breaks.

[bitprophet]

See #357 :)

[rsxm]

Thank you! Don't know how I missed that one, I thought I was thorough with my search of existing issues :-)