Add support for overloaded functions

[ketozhang]

Currently I can't seem to get sphinx-autodoc2 to generate the signature for overloaded functions.

Example

from typing import overload

@overload()
def myfunc(a: int):
  """foobar with int"""
...

@overload()
def myfunc(a: str):
  """foobar with str"""
...

def myfunc(a):
  """foobar"""
  # Implementation