Support specifying headers per-request

[kedarv]

In Yelp/bravado, developers can specify per-request configuration by passing in _request_options to the service call. A potential usecase is specifying headers which are consumed by the downstream service (or services in the middle of the request path), such as Yelp/casper.

The python api looks like:

client = SwaggerClient.from_url(...)
request_options = { ... }
client.pet.getPetById(petId=42, _request_options=request_options).response().result

A potential solution is to expose a similarly named parameter in the JS equivalents.

[magicmark]

@kedarv can you sketch out what you expect the desired API to look like?