Make the clipboard command user-configurable

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With this simplification, the old OSX test is obsolete, and new OSX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).

rtv/templates/rtv.cfg

@@ -16,6 +16,12 @@ ascii = False
 ; Turn on monochrome mode to disable color.
 monochrome = False
 
+; Data being copied is piped into this command
+;clipboard_cmd = xclip
+;clipboard_cmd = xsel -b -i
+;clipboard_cmd = wl-copy
+;clipboard_cmd = pbcopy w
+
 ; Flash when an invalid action is executed.
 flash = True