More tests...
Interactive history search fixes and tests

Fixed some very minor issues with interactive history search including:
    * Actually work if started with a forward search
    * Support Backspace even if bound to a custom command
    * Remove emphasis when search is in a failed state
    * Don't use current line as search text when starting search

Tests added cover close the most important stuff, when I get closer
to 100% coverage I'll add a few more tests.
Update comment in Ctrl+R test
WhatIsKey tests
Remove custom bindings
Fix WriteLine to properly scroll
ShowKeyBindings test
Always reset bindings when setting edit mode

As a misguided optimization, if the edit mode is specified
but not changing, we skipped setting the key bindings.
CharacterSearch/CharacterSearchBackward tests
YankLastArg/YankNthArg tests and minor fixes
Cut/Copy/Paste tests
InvokePrompt tests

Removed Console.WriteLine replacement as it didn't work for InvokePrompt.
Changed test harness to override VS and actually write stdout to the screen.
PublicAPI tests