Provide graceful shutdown capability; optionally hide Start at Login menu item

[forfuncsake]

For a private project I'm working on, I am running a background server along with the menu app.

• We sometimes need to start the application as root, which makes the Start at Login not work as expected - so allow it to be hidden.
• We want to gracefully shutdown the background service when the menu bar app quits - so provide optional hooks into the terminate flow.

[caseymrm]

Looks great, thanks for your contribution!

[forfuncsake]

Thanks for merging - would you mind please adding a new version tag so we can use the new features in a go module version? Thx.