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With this simplification, the old OSX test is obsolete, and new OSX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/page.py

@@ -542,7 +542,7 @@ class Page(object):
             return
 
         try:
-            clipboard_copy(url)
+            clipboard_copy(url, self.config['clipboard_cmd'])
         except (ProgramError, OSError) as e:
             _logger.exception(e)
             self.term.show_notification(