Add command-line options to force-enable/disable color

`-forcecolor` will force color to be on. `-nocolor` will force color to
be off.

And just because I'm a nice person, I've also added British versions of
those flags. As a treat.

desktop_version/src/Vlogging.h

@@ -8,6 +8,8 @@ extern "C"
 
 void vlog_init(void);
 
+void vlog_toggle_color(int enable_color);
+
 int vlog_info(const char* text, ...);
 
 int vlog_warn(const char* text, ...);