COMMON: Introduce --debug-channels-only command line flag.

Many of our systems currently generate significant amount of debug
output on deeper levels. Now, when your engine is using Debug Channels,
you might want to show that debug information only, which is currently
not possible, as the generic output will be mixed in your output.

Alternative solution would be to implement possibility to specify
per-channel debug levels.

base/commandLine.cpp

@@ -97,6 +97,7 @@ static const char HELP_STRING[] =
 	"  -d, --debuglevel=NUM     Set debug verbosity level\n"
 	"  --debugflags=FLAGS       Enable engine specific debug flags\n"
 	"                           (separated by commas)\n"
+	"  --debug-channels-only    Show only the specified debug channels\n"
 	"  -u, --dump-scripts       Enable script dumping if a directory called 'dumps'\n"
 	"                           exists in the current directory\n"
 	"\n"
@@ -426,6 +427,9 @@ Common::String parseCommandLine(Common::StringMap &settings, int argc, const cha
 			DO_LONG_OPTION("debugflags")
 			END_OPTION
 
+			DO_LONG_OPTION_BOOL("debug-channels-only")
+			END_OPTION
+
 			DO_OPTION('e', "music-driver")
 			END_OPTION