Allow custom headers

[ptrckvzn]

Either in config or in fetch request, the client should accept custom headers.

Right now it is not possible to pass a Accept-Language header to the client.

https://github.com/area17/twill-api-client/blob/main/src/index.ts#L29

and possibly per request: $twill.find('pages').headers({ 'Accept-Language': 'fr' }).fetch().