feature suggestion: bud run should launch browser (maybe with a flag to enable?)

[sneak]

When launching the dev server, I have to manually open a browser tab and load the URL. It would be cool if it called out to the OS and simply told it to open the dev server URL. The quick and dirty (on macOS) is to shell out to open $URL but I imagine there's a go module somewhere that does this.

Even a flag to do it would be fine, as I invoke bud run from a Makefile (make dev).

c.f.: https://talk.jekyllrb.com/t/automatically-start-browser-on-localhost-4000-after-jekyll-serve/4916/5

[matthewmueller]

Hey @sneak, I'd definitely accept a PR with an --open flag! I've used this one in the past: https://github.com/pkg/browser