settings: make characters used in visualizer customizable

doc/ncmpcpp.1

@@ -99,6 +99,9 @@ Defines interval between syncing visualizer and audio outputs.
 .B visualizer_type = spectrum/wave
 Defines default visualizer type (spectrum is available only if ncmpcpp was compiled with fftw support).
 .TP
+.B visualizer_look = STRING
+Defines visualizer's look (string has to be exactly 2 characters long: first one is for wave whereas second for frequency spectrum).
+.TP
 .B system_encoding = ENCODING
 If you use encoding other than utf8, set it in order to handle utf8 encoded strings properly.
 .TP