v0.3.8

• Add "extended CONNECT support". Adds h2::ext::Protocol, which is used for request and response extensions to connect new protocols over an HTTP/2 stream.
• Add max_send_buffer_size options to client and server builders, and a default of ~400KB. This acts like a high-water mark for the poll_capacity() method.
• Fix panic if receiving malformed HEADERS with stream ID of 0.