feat: allow formatDate and formatTime to take string type (#1369)

JS have implicitly type conversion, 2019-07-13 can be read correctly.
This makes it easy to use when date is read from a JSON object.

Author: leoyli

src/types.ts

@@ -59,8 +59,8 @@ export type MessageFormatPrimitiveValue =
   | undefined;
 
 export interface IntlFormatters {
-  formatDate(value: number | Date, opts: FormatDateOptions): string;
-  formatTime(value: number | Date, opts: FormatDateOptions): string;
+  formatDate(value: string | number | Date, opts: FormatDateOptions): string;
+  formatTime(value: string | number | Date, opts: FormatDateOptions): string;
   formatRelativeTime(
     value: number,
     unit?: FormattableUnit,