Support for additional methods

[mogelbrod]

I'd love to replace the bloated mixpanel-browser package with this one. Unfortunately my team currently depend on the following APIs for our app:

• mixpanel.people.set_once()
• mixpanel.time_event()
• mixpanel.has_opted_in_tracking()
• mixpanel.opt_in_tracking() (similar to mute/unmute but without the logging)

We're lacking the resources to dive into the codebase right now, but figured filing an issue could be of some worth. If somebody considers these additions worthwhile (and don't end up implementing it themselves) I might be able to dive into it at a later time.