Allow forcing of colored output

In some cases (ie drone ci) there is no tty available but its still
possible to display colors in the webui.

functions.sh.in

@@ -243,7 +243,9 @@ enable_colors() {
 	BLUE="\033[1;34m"
 }
 
-if [ -n "$USE_COLORS" ] && [ -t 1 ]; then
+if [ "$USE_COLORS" = force ]; then
+	enable_colors
+elif [ -n "$USE_COLORS" ] && [ -t 1 ]; then
 	enable_colors
 else
 	disable_colors