pass more information to `Proxy.DialTarget`

[marten-seemann]

Applications might want to know which client they're establishing a connection for, for example.

[marten-seemann]

With #40, we now have two ways to achieve this:

1. Attach information to the DialTarget context.
2. Add a new parameter to the DialTarget callback.

[marten-seemann]

Actually, the application can do it all by itself. This is pretty neat!

Here's how it works:

• The applications accepts the request in its own HTTP handler, and then calls the proxy's Upgrade function. Before doing so, it can attach any value it likes to the request context.
• It can then retrieve this value from the context on the DialTarget callback.