Improve specrum visulaizer for transparent backgrounds (#467)

* Replace "hack" with hacky chars

* Remove unnecessary spaces

* Add config option visualizer_spectrum_use_legacy_chars

src/settings.cpp

@@ -253,6 +253,7 @@ bool Configuration::read(const std::vector<std::string> &config_paths, bool igno
 			});
 	p.add("visualizer_autoscale", &visualizer_autoscale, "no", yes_no);
 	p.add("visualizer_spectrum_smooth_look", &visualizer_spectrum_smooth_look, "yes", yes_no);
+	p.add("visualizer_spectrum_use_legacy_chars", &visualizer_spectrum_use_legacy_chars, "yes", yes_no);
 	p.add("visualizer_spectrum_dft_size", &visualizer_spectrum_dft_size,
 			"2", [](std::string v) {
 			auto result = verbose_lexical_cast<size_t>(v);