A command argument can now be completed even if it isn’t the last
one in the input.
- Add a new method Input.new_completion
Almost like the old auto_completion method, except taht it takes
another argument: argument_position, which is the argument to be
completed.
- Methods using the old completion method still work
- All completion methods in poezio now use the new one if necessary
- Further details can be found in the docstring of new_completion
