Keybindings for d3
Demo: http://bl.ocks.org/4444952
# d3.keybinding()
Returns a keybinding object that can be used with .call to provide
bindings on selections.
The keybinding object exposes .on() for common keycodes and dispatches events
with arguments [d3.event, modifiers] in which modifiers is a string which
can include ⇧, ⌃, ⌥, ⌘'.
Keystroke events on input areas (<input>, <select>, and <textarea>)
are not caught.