Add xclip.el.
* xclip.el: Better follow conventions.
Fix up copyright notice.
(xclip-program): Make it work in the usual way.
(xclip-set-selection, xclip-selection-value): Obey xclip-program.
(turn-on-xclip, turn-off-xclip): Don't autoload, not interactive.
(xclip-mode): New minor mode to avoid enabling it unconditionally.