Add coloured logs (#4549)

The setting log_colour can be used to en-/disable or autodetect it.

builtin/settingtypes.txt

@@ -1207,6 +1207,13 @@ language (Language) enum   ,be,ca,cs,da,de,en,eo,es,et,fr,he,hu,id,it,ja,jbo,ko,
 #    -    verbose
 debug_log_level (Debug log level) enum action ,none,error,warning,action,info,verbose
 
+#    ANSI colored logs: red error log, yellow warning and grey info and verbose logs
+#    Note that it doesn't work on Windows
+#    "yes" always enables it,
+#    "detect" enables it when printing to terminal and
+#    "no" disables it
+log_color (Colored logs) enum detect yes,detect,no
+
 #    IPv6 support.
 enable_ipv6 (IPv6) bool true