- (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

src/post-processors/post-processor.interface.ts

@@ -0,0 +1,9 @@
+import { TranslationCollection } from '../utils/translation.collection';
+
+export interface PostProcessorInterface {
+
+	name: string;
+
+	process(working: TranslationCollection, extracted: TranslationCollection, existing: TranslationCollection): TranslationCollection;
+
+}