Add functional options to allow configuring the underlying Cmd

[landism]

Problem

browser exposes the Stdout and Stderr vars for consuming packages to control where its output goes. This global state serves most purposes, but doesn't suffice if a project wants different output destinations for different OpenURL calls.

Solution

Add options to the Open* functions to allow the user to configure the command's stdout and stderr.

Notes

• It seems unfortunate that cmdOptions will be ignored on Windows, but the same was already true of the package's Stdout/Stderr vars. It seems quite possible the syscall used on windows doesn't have output anyway, but I'm not very familiar with it.
• It's possible that changing stdout/stderr is really the only interesting thing one might want to do with CmdOption, so maybe it'd make more sense to just define an OpenOption type that just has stdout/stderr and keep exec.Command out of the API.
• The changes to go.mod/go.sum occurred when I built the package with GOOS=windows. It looks like they should have been included in Windows: utilize ShellExecuteW #27. If you'd like, I'm happy to put them up as a separate PR since they're not really part of this change, or they're also taken care of by Fix running project on Windows #29, if that ever gets merged.