fixes #175 by allowing full options inclusion in status and fixing types in createResponse

[kwhitley]

Description

1. New signature for status takes full ResponseInit options as second param:

   status = (status: number, options?: ResponseInit): Response

2. Changed ResponseFormatter internal type for createResponse to be more helpful in downstream uses (e.g. json)

   interface ResponseFormatter {
     (body?: any, options?: ResponseInit): Response
   }

Issue Related

See #175

TODO

• add documentation to itty.dev