Add ClearScreen

ClearScreen clears the screen then redraws the current line at the top of the screen.
Merge branch 'master' of https://github.com/lzybkr/PSReadLine
Fix bug where console paste mixes up keys

Console paste was broken by the fix to exit w/o delay because
I needed to reconsider the logic of ReadKey instead of just refactoring
the code onto another thread.