[type]: fix 'initialIndex' type (#142)

* [type]: fix initialIndex type

* remove redundant 'undefined' type

Author: ahmadi-akbar

src/index.d.ts

@@ -55,9 +55,9 @@ export interface FlickityOptions {
     cellSelector?: string;
 
     /**
-     * Zero-based index of the initial selected cell.
+     * Zero-based index or selector string of the initial selected cell.
      */
-    initialIndex?: number;
+    initialIndex?: number | string;
 
     /**
      * Enable keyboard navigation. Users can tab to a Flickity gallery, and pressing left & right keys to change cells.
@@ -178,7 +178,7 @@ export interface FlickityOptions {
      * 
      * default: false
      */
-     pauseAutoPlayOnHover?:boolean;
+     pauseAutoPlayOnHover?: boolean;
 
     /**
      * Changes height of carousel to fit height of selected slide.