support for adding items like in ncmpc (disabled by default)

doc/ncmpcpp_keys

@@ -37,6 +37,8 @@
 #
 #key_toggle_space_mode = 't'
 #
+#key_toggle_add_mode = 'T'
+#
 #key_screen_switcher = 9
 #
 #key_help = '1' 265

doc/ncmpcpprc

@@ -94,7 +94,7 @@
 #
 ##### various settings #####
 #
-#playlist_display_mode = "normal" (classic/columns)
+#playlist_display_mode = "classic" (classic/columns)
 #
 #incremental_seeking = "yes"
 #
@@ -118,6 +118,8 @@
 #
 #follow_now_playing_lyrics = "no"
 #
+#ncmpc_like_songs_adding = "no" (enabled - add/remove, disabled - always add)
+#
 #enable_window_title = "yes"
 #
 ##### colors definitions #####