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.c

@@ -47,6 +47,11 @@ void vlog_init(void)
     }
 }
 
+void vlog_toggle_color(const int enable_color)
+{
+    color_enabled = enable_color;
+}
+
 int vlog_info(const char* text, ...)
 {
     va_list list;