Implement Headers type in TypeScript definition
Implement Headers type in TypeScript definition

Usage of headers was documented in the readme but not included in the TypeScript definitions. This fixes that.

As a work around I added this locally to my project for testing.
```ts
declare module 'wikijs' {
  /**
   * The Headers sent to be sent along with the request
   *
   * @type {HeadersInit}
   * @memberof Options
   */
  interface Options {
    headers: HeadersInit;
  }
}
```
dijs/wiki