Automatically expand python parameters

Hi, this is a first stab to automatically expand the parameters of a python callable when invoked. Basically, whenever the opening parenthesis of a call is typed, like `object.some_method(_`, the expansion snippet is triggered.

By using the opening parenthesis as a trigger, parameter expansion will not get in the way when passing functions as parameters, such as in `map(object.some_method, [1, 2, 3])`.

For this to work in languages where overloading methods is possible, I guess the completer needs to know which index was selected in the popup menu.

Anyway, let me know what you think about this approach.