implement custom logging formatter

supports custom log message formats for each loglevel and, by
extension, custom ANSI codes and colors for errors and warnings

(#304)

docs/gallery-dl-example.conf

@@ -148,8 +148,13 @@
     {
         "mode": "terminal",
         "log": {
-            "format": "{name}: {message}",
-            "level": "info"
+            "level": "info",
+            "format": {
+                "debug"  : "\u001b[0;37m{name}: {message}\u001b[0m",
+                "info"   : "\u001b[1;37m{name}: {message}\u001b[0m",
+                "warning": "\u001b[1;33m{name}: {message}\u001b[0m",
+                "error"  : "\u001b[1;31m{name}: {message}\u001b[0m"
+            }
         },
         "logfile": {
             "path": "~/gallery-dl/log.txt",