Added ability to include directories as an extension of the config file (#421)

config.example.toml

@@ -1,3 +1,10 @@
+# Include any additional configuration file(s)
+# [include] sections are processed in the order you write them
+# Files in $CONFIG_DIR/topgrade/topgrade.d/ are automatically included at the beginning of this file
+[include]
+#paths = ["/etc/topgrade.toml"]
+
+[misc]
 # Don't ask for confirmations
 #assume_yes = true