crypto/tls: add DialWithDialer.

crypto/tls: add DialWithDialer.

While reviewing uses of the lower-level Client API in code, I found
that in many cases, code was using Client only because it needed a
timeout on the connection. DialWithDialer allows a timeout (and
 other values) to be specified without resorting to the low-level API.

LGTM=r
R=golang-codereviews, r, bradfitz
CC=golang-codereviews
https://golang.org/cl/68920045