Implementation of the yank-nth-arg and yank-last-arg Readline commands + Emacs bindings.

This adds the following Emacs key bindings:
- meta-.     : yank-last-arg
- meta-_     : yank-last-arg
- meta-CtrlY : yank-nth-arg

Thanks a lot to Marcin Bachry (@mbachry) and Adam Chainz (@adamchainz)
for creating a proof of concept implementation and pull request. See:
- https://github.com/jonathanslenders/python-prompt-toolkit/pull/407
- https://github.com/jonathanslenders/python-prompt-toolkit/pull/336