Add visualizer_spectrum_gain option to a configuration file

doc/config

@@ -111,6 +111,8 @@
 #
 #visualizer_spectrum_dft_size = 2
 #
+#visualizer_spectrum_gain = 20
+#
 ## Left-most frequency of visualizer in Hz, must be less than HZ MAX
 #
 #visualizer_spectrum_hz_min = 20

doc/ncmpcpp.1

@@ -109,6 +109,9 @@ For spectrum visualizer, use unicode block characters for a smoother, more conti
 .B visualizer_spectrum_dft_size = NUMBER
 For spectrum visualizer, a value between 0 and 4 inclusive. Specifying a larger value makes the visualizer look at a larger slice of time, which results in less jumpy visualizer output.
 .TP
+.B visualizer_spectrum_gain = dB
+Gain for spectrum visualizer in dB, larger/smaller values shift bars up/down.
+.TP
 .B visualizer_spectrum_hz_min = Hz
 For spectrum visualizer, left-most frequency of visualizer, must be less than HZ MAX.
 .TP