Support for accessing req and res plumber objects inside porcelain endpoints

[hillalex]

Given an instance of porcelain_endpoint, the plumber request handler that is run under the hood does not pass the entire plumber request to the endpoint handler, but rather extracts just the path args, query parameters and body and passes them along.

But sometimes it is useful to have access to the whole req object; for example, if using the plumber session_cookie hook to attach sessions to the request (available at req$session).

We could try and identify the extra parts of the request that a user might want and add them individally, as per the path args, query strings and body, but a simplier and more flexible solution would just be to allow porcelain endpoint handlers to optionally accept a req argument.

[hillalex]

@richfitz I've opened a PR adding this functionality #40 - would you be happy to accept this change?

[richfitz]

OMG sorry I missed the original issue entirely (though tbf I was on Lundy at the time I think!)

[hillalex]

No worries! I've been using porcelain via a fork, but would be great to get this in...