add types

src/compilers/json.compiler.ts

@@ -7,7 +7,7 @@ export class JsonCompiler implements CompilerInterface {
 
 	public indentation: string = '\t';
 
-	public extension = 'json';
+	public extension: string = 'json';
 
 	public constructor(options?: any) {
 		if (options && typeof options.indentation !== 'undefined') {

src/compilers/po.compiler.ts

@@ -5,12 +5,12 @@ import * as gettext from 'gettext-parser';
 
 export class PoCompiler implements CompilerInterface {
 
-	public extension = 'po';
+	public extension: string = 'po';
 
 	/**
 	 * Translation domain
 	 */
-	public domain = '';
+	public domain: string = '';
 
 	public constructor(options?: any) {}