This change adds a Dialer field to websocket.Config struct. If this
value is set the Dialer will be used. If it's nil, DialConfig will
create an empty Dialer that will maintain the original behavior.

Because before Go 1.3 there was no crypto/tls.DialWithDialer function,
the Dialer will be ignored when opening TLS connections in these
versions.

Fixes golang/go#9198.

Change-Id: If8b5c3c47019a3d367c436e3e60eb54bf0276184
Reviewed-on: https://go-review.googlesource.com/12191Reviewed-by: Russ Cox <rsc@golang.org>