allow for empty tag marker customization

doc/config

@@ -176,6 +176,8 @@
 #
 #mouse_support = "yes"
 #
+#empty_tag = "<empty>"
+#
 #enable_window_title = "yes"
 #
 ##

doc/ncmpcpp.1

@@ -212,6 +212,9 @@ Number of lyrics database in use. Currently supported DBs are:
 .B media_library_left_column = a/y/g/c/p
 Default tag type for left column in media library. Legend for possible letters is in  SONG FORMAT section.
 .TP 
+.B empty_tag = TEXT
+Text that will be displayed, if requested tag is not set.
+.TP 
 .B empty_tag_color = COLOR
 Color of empty tag marker.
 .TP 

src/settings.cpp

@@ -712,6 +712,10 @@ void ReadConfiguration(ncmpcpp_config &conf)
 				if (!v.empty())
 					conf.song_window_title_format = v;
 			}
+			else if (cl.find("empty_tag") != std::string::npos)
+			{
+				conf.empty_tag = v; // is this case empty string is allowed
+			}
 			else if (cl.find("empty_tag_color") != std::string::npos)
 			{
 				if (!v.empty())