Allow herd open to retain focus in the terminal

[chris-ware]

I'd like to suggest an extra parameter be added to the herd open command that keeps the focus in the terminal rather than directing users to the browser. Assuming that the underlying code runs open on MacOS systems, passing the -g parameter into this command will do this. This allows for some tooling that wraps around herd commands, such as an end-to-end site initialisation script, to retain focus and allow for prompts to continue without needing the user to refocus back on their terminal.