Add config item clipboard_cmd, with a default of 'pbcopy w' on Darwin
and 'xclip' on everything else. This will allow the user to use any
command for the clipboard, including 'wl-copy' for Wayland (addressing
issue #693 on Github). With his change, significant simplifications
could be made to clipboard.py - the copy_*() functions have been removed
and combined into copy().
With this simplification, the old OSX test is obsolete, and new OSX
tests are needed (need a way to simulate sys.platform).
