Allow shifted cursor keys to be used to select text.

Change modifier key for `windmove`, so we can use shifted cursor keys
for selecting text. This has to be done before loading Prelude.
See https://github.com/bbatsov/prelude/issues/491 for more details.
Move Emacs keymap config to its own file.

Make sure all the cursor keys work from various terminals.
These should cover just about any terminal emulator.
This includes cursor keys with modifier keys as well.