Update numeraljs.d.ts

Added formatCurrency, changed format to a optional param to match functionality and spec.

numeraljs/numeraljs.d.ts

@@ -27,7 +27,8 @@ interface Numeral {
 	language(key: string, values?: NumeralJSLanguage): Numeral;
 	zeroFormat(format: string): string;
 	clone(): Numeral;
-	format(inputString: string): string;
+	format(inputString?: string): string;
+	formatCurrency(inputString?: string): string;
 	unformat(inputString: string): number;
 	value(): number;
 	valueOf(): number;