Add time getter setter

[marwie]

Hello,

We started using quarks two days ago and it's awesome so far! 😊 We're using it in our threejs based engine with data exported directly from Unity (you can see a live example rendered using quarks here if you're curious)

For playing the particle effect with burst values again (and have quarks call the behaviours again after the system has reached the end time) I need to reset the time.
This PR exposes accessors for the internal emission state.

[Alchemist0823]

Thank you marwie,

I am glad to hear that. would like to check out your exporter. could you give attribution of this engine somewhere in your product? MIT license requires attribution.