music: add the possibility to raise music player

lib/spotify.lua

@@ -35,3 +35,9 @@ end
 function previous()
    spotify("Previous")
 end
+
+function show()
+   -- This should work, but no:
+   -- spotify("Raise")
+   awful.util.spawn("spotify")
+end

rc/bindings.lua

@@ -231,6 +231,7 @@ config.keys.global = awful.util.table.join(
                                   elseif key == "c" then music.pause()
                                   elseif key == "v" then music.stop()
                                   elseif key == "b" then music.next()
+                                  elseif key == "s" then music.show()
                                   end
                                   return true
                                end)