Support 2 key chords

Support for 2 key chords has been added.

There is a small breaking change in Set-PSReadlineKeyHandler
because the -CtrlX switch is no longer needed.  It could have
been left in, but it's better to just remove it - not very
many people use this module yet.

The signature of key handlers was also changed to better support
features that have not yet been implemented.  The signature change
shouldn't affect any existing key handlers because default arguments
should cover those cases.
Fix cut and paste error that broke Ctrl-C