silk middleware

As a Developer
I want to Record tests to silk files
As I manually test my application.

Concept
Provide a middleware handler(s) with flags to enable a record mode and options. The middleware would generate a silk compatible file including the HTTP method and related request parameters, headers, status and response. Start with the standard library http, then adapt to specific frameworks that have more advanced routers.

Proof of Concept
Handle POST request and response for a form. Produce an acceptable set of silk test document(s) for a series of requests.

Start with the Go standard-library HTTP middleware entry point and the Silk document format. Build a proof of concept around a POST form request, recording the method, request parameters, headers, status, and response. Done means a series of recorded requests produces acceptable Silk test documents.

Written by the indexing model from the issue text.