cmd/pprof/internal/commands: add command to open browser on windows

While we're at there, also add a message to prompt the user to install
Graphviz if "dot" command is not found.

Fixes issue 9178.

LGTM=adg, alex.brainman, cookieo9, rsc
R=rsc, adg, bradfitz, alex.brainman, cookieo9, smyrman
CC=golang-codereviews
https://codereview.appspot.com/180380043

Committer: Russ Cox <rsc@golang.org>