Allow overriding default net.Dialer

[fidraC]

Not sure if this is appropriate but I had a situation pop up where I needed to define my own net.Dialer so that I could override the default DNS resolver.

[bogdanfinn]

@fidraC thank you for your contribution! For me that looks good so far.
What do you think about using this option also in the newConnectDialer() not only in the directDialer ?
Does that make sense that someone maybe wants to override the underlying net.dialer there aswell?

[fidraC]

What do you think about using this option also in the newConnectDialer() not only in the directDialer ?

This makes sense that if someone wants to override the default dialer, they would want to override all dialers for the particular client. I'll look through the code and see what needs to be changed.

[fidraC]

done!