Allow caller to deal with failure #15

Please update the "unsupported" case to be more useful. You could provide a browser.IsSupported() that returns true only on supported platforms.

I find the use of build-time handling of different platforms to be overly complex. After reviewing this module and considering writing a pull request, I just went with:

func openBrowser(url string) {
	cmd := "xdg-open"
	args := []string{url}

	switch runtime.GOOS {
	case "darwin":
		cmd = "open"
	case "windows":
		cmd = "rundll32"
		args = []string{"url.dll,FileProtocolHandler", url}
	}
	err := exec.Command(cmd, args...).Start()
	if err == nil {
		return
	}
	fmt.Printf("Failed to launch %s: %v", cmd, err)
	fmt.Printf("Load this URL in your browser:\n    %s\n", url)
}

because it could work on more types of Unix systems and more gracefully fails when assumptions are not met.

Adding support for the BROWSER environment variable to that would be even better.