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.cpp

@@ -42,7 +42,8 @@ static const std::string item_type_strings[] = {
 	"rule output",
 	"rule output expression",
 	"rule priority",
-	"overrides"
+	"overrides",
+	"extension item"
 };
 
 const std::string& rule_loader::context::item_type_as_string(enum item_type it)