rename empty_tag config option to empty_tag_marker

it was conflicting with empty_tag_color. since empty_tag_color
contains empty_tag, ncmpcpp was parsing wrong variable if the
first variable was before the second one in configuration file.

doc/config

@@ -226,7 +226,7 @@
 #
 #mouse_support = "yes"
 #
-#empty_tag = "<empty>"
+#empty_tag_marker = "<empty>"
 #
 #enable_window_title = "yes"
 #

doc/ncmpcpp.1

@@ -238,7 +238,7 @@ Default user interface used by ncmpcpp at start.
 .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
+.B empty_tag_marker = TEXT
 Text that will be displayed, if requested tag is not set.
 .TP 
 .B empty_tag_color = COLOR

src/settings.cpp

@@ -817,7 +817,7 @@ void ReadConfiguration(ncmpcpp_config &conf)
 					conf.song_window_title_format += '}';
 				}
 			}
-			else if (cl.find("empty_tag") != std::string::npos)
+			else if (cl.find("empty_tag_marker") != std::string::npos)
 			{
 				conf.empty_tag = v; // is this case empty string is allowed
 			}