bindings: add a binding to spawn pavucontrol

lib/spotify.lua

@@ -74,3 +74,7 @@ end
 function previous()
    cmd("Previous")
 end
+
+function mixer()
+   awful.util.spawn("pavucontrol")
+end

rc/bindings.lua

@@ -271,6 +271,7 @@ config.keys.global = awful.util.table.join(
                                   elseif key == "v" then music.stop()
                                   elseif key == "b" then music.next()
                                   elseif key == "s" then music.show()
+                                  elseif key == "m" then music.mixer()
                                   end
                                   return true
                                end)