Add addl support for rules reader/compiler subclasses

To support subclasses that may extend the falco rules format, add
additional error/warning/item types for an extension item.

When subclasses report errors and warnings, they can use these
codes/item types in context objects and still provide an exact
line/column context.

Also make some previously static functions in rules reader protected
methods so they can be used in sub-classes.

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>

userspace/engine/rule_loader.h

@@ -58,7 +58,8 @@ namespace rule_loader
 			RULE_OUTPUT,
 			RULE_OUTPUT_EXPRESSION,
 			RULE_PRIORITY,
-			OVERRIDE
+			OVERRIDE,
+			EXTENSION_ITEM
 		};
 
 		static const std::string& item_type_as_string(enum item_type it);