Add two flags EnableRequests and EnableResponses to DefaultFormatter, which enable printing http.Request and http.Response on failure. New flags should be disabled by default.

Request and response instances are already present in AssertionContext passed to formatter. Formatted request and response should be added to FormatData and used in failure template. Formatting can be done using httputil, like we do it in DebugPrinter.

Formatted response should include round trip time (again like in DebugPrinter).

Related discussion: #137

See also #160.