- (feat) add concept of post processors

- (feat) add 'key as default value' post processor (closes #109)
- (chore) move clean functionality to a post processor
- (chore) move sort functionality to a post processor
- (refactor) get rid of leading underscore on protected properties/methods

tests/parsers/directive.parser.spec.ts

@@ -5,7 +5,7 @@ import { DirectiveParser } from '../../src/parsers/directive.parser';
 class TestDirectiveParser extends DirectiveParser {
 
 	public normalizeTemplateAttributes(template: string): string {
-		return this._normalizeTemplateAttributes(template);
+		return super.normalizeTemplateAttributes(template);
 	}
 
 }