Implicit WebSocket support

[mpapierski]

This might be good idea to add implicit support for WebSocket.

Plan:

• Add "on_upgrade" callback for user that will allow or disallow further processing of possible WebSocket connection
• Add "on_error" callback to detect possible disconnection of client
• After upgrading "on_body" should be called with received WebSocket frames
• User should be aware that the client is upgraded to WebSocket or not (For example: client->is_websocket).
• Improve "pausing" API to allow pausing after receiving chunk of body to allow further "background processing"
• Allow possible "opt-out" by disabling WebSocket during compile time.

Possible implementations:

• https://github.com/tatsuhiro-t/wslay - This library seems to be good at embedding