Expose the Dialer instance

[kubuzetto]

I applied a change that exposes the net.Dialer to be used for the connection via the Opts struct. Instead of using net.DialTimeout, the users of the library can provide a custom Dialer to control more aspects of the connection step than just the timeout.

• Passing a simple Dialer interface here allows one to mock the actual connection part; making unit tests easier.
• The users can implement custom dialers to extend the behavior, for example a "proxy dialer" can allow the library to direct the traffic through proxies (this is my use case, and the reason why I'm opening this PR)