Store connection details

[SpikeHD]

I am working on a project that requires information from the query parameters and headers of the websocket connection, and I'm in too deep with this library to want to switch :P (and because it's just so... simple!).

This PR introduces a new struct, ConnectionDetails that contains both the URI as a String and the headers as a tungstenite HeaderMap. This shouldn't break the API, so anyone who updates from 0.1.6 to this shouldn't have to refactor anything