Refactored InteractiveShell's "handleKeyPress" method, added two new hotkeys.

I am open to suggestions for key combinations! Thus far, the new
hotkeys are:

SHIFT + ENTER: evaluate code in both single- and multi-line input modes.

CTRL + /: Switches between single- and multi-line input modes,
depending upon the state of the text box. Be careful, this doesn't
(currently) do anything special if you press CTRL+/ and there is text
in the field…you may lose it.

Previously existing hotkeys and their behaviors are unchanged.

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