cmd/go: make bug subcommand open the browser

cmd/go: make bug subcommand open the browser

Instead of dumping information for the use
to copy/paste into the issue tracker,
open the issue tracker directly with a pre-filled
template.

Change-Id: I370d0063b609200497014ccda35244fa4314a662
Reviewed-on: https://go-review.googlesource.com/29210
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>