Autoclick

* Automatically add version option `version` argument to command/group decorator is not `None`

* Remove `extra_click_kwargs` parameter to command/group decorator - instead, any extra keyword arguments to the decorator are passed to the Command/Group initializer.
* Add ability to automatically run a command/group when a module is called as an executable (i.e. when __name__ == "__main__") by setting `main=True` in the command/group decorator.

* Remove support for *args and **kwargs (these never worked properly anyway)
* Add support for aggregate types
* Implement auto-conversion for dict types
* Enable type conversions to be used generically - conversions with type parameters can optionally match the generic type

* Fix error due to difference between python 3.6 and 3.7

* Add additional types
* Add version option
* EnumChoice: don't try to convert if the value is already an enum

* Fix bug in get_conversion in which wrong type was being used to look up automatic conversion function