fix(typescript): clarify input to parseIndex

Narrowing the type of the parameter from `any` makes it obvious that the
input is an `Object`, rather than a `String`.

With the `any` type as input, it was not obvious how to use this
function.

See krisk#524 and krisk#624 for people bitten by this.

Author: dbaynard

src/index.d.ts

@@ -91,7 +91,10 @@ declare class Fuse<T> {
   ): FuseIndex<U>
 
   public static parseIndex<U>(
-    index: any,
+    index: {
+      keys: ReadonlyArray<string>
+      records: FuseIndexRecords
+    },
     options?: FuseIndexOptions<U>
   ): FuseIndex<U>