Add i18nFormat configuration to InitOptions (#29536)

types/i18next/index.d.ts

@@ -5,6 +5,7 @@
 //                 Giedrius Grabauskas <https://github.com/GiedriusGrabauskas>
 //                 Silas Rech <https://github.com/lenovouser>
 //                 Philipp Katz <https://github.com/qqilihq>
+//                 Milan Konir <https://github.com/butchyyyy>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -385,6 +386,12 @@ declare namespace i18next {
          * Compatibility JSON version
          */
         compatibilityJSON?: string;
+
+        /**
+         * options for i18n nessage format - check documentation of plugin
+         * @default undefined
+         */
+        i18nFormat?: object;
     }
 
     // Add an indexer to assure that interpolation arguments can be passed