command: add an ab_loop command

As suggested in #1241; to make using the feature easier.

Also add better OSD-formatting for the ab-loop-a/b properties.

input/cmd_list.c

@@ -155,6 +155,8 @@ const struct mp_cmd_def mp_cmds[] = {
 
   { MP_CMD_DISCNAV, "discnav", { ARG_STRING } },
 
+  { MP_CMD_AB_LOOP, "ab_loop", },
+
   { MP_CMD_AF, "af", { ARG_STRING, ARG_STRING } },
   { MP_CMD_AO_RELOAD, "ao_reload", },