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With this simplification, the old OSX test is obsolete, and new OSX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

tests/test_clipboard.py

@@ -3,7 +3,7 @@ from __future__ import unicode_literals
 
 import pytest
 
-from rtv.clipboard import copy_linux, copy_osx
+from rtv.clipboard import copy
 from rtv.exceptions import ProgramError
 
 
@@ -23,20 +23,13 @@ def test_copy():
         p.communicate = mock.Mock()
         Popen.return_value = p
 
-        # If the `which` command can't find a program to use
-        call.return_value = 1  # Returns an error code
-        with pytest.raises(ProgramError):
-            copy_linux('test')
-
         call.return_value = 0
-        copy_linux('test')
+        copy('test', 'xsel -b -i')
         assert Popen.call_args[0][0] == ['xsel', '-b', '-i']
         p.communicate.assert_called_with(input='test'.encode('utf-8'))
-        copy_linux('test ❤')
+
+        copy('test ❤', 'xclip')
+        assert Popen.call_args[0][0] == ['xclip']
         p.communicate.assert_called_with(input='test ❤'.encode('utf-8'))
 
-        copy_osx('test')
-        assert Popen.call_args[0][0] == ['pbcopy', 'w']
-        p.communicate.assert_called_with(input='test'.encode('utf-8'))
-        copy_osx('test ❤')
-        p.communicate.assert_called_with(input='test ❤'.encode('utf-8'))
+    # Need OSX tests, can't simulate sys.platform