Add a ReadlineMixin class to emulate some of readline behavior

Here is a mixin class to add common readline shortcuts to Edit widgets.

It contains tests, doc and an example that I put in the examples/ directory. One might argue that the example is a bit too simple to be there - please give me your feedback.

I think it is compatible with the "highlights" but if I understand correctly, the latter is not working at the moment I could not really test it.

Cheers,
Sitaktif