Support pausing/resuming effect queue processing

[delucis]

As discussed in #85, in some scenarios a client may want to pause and then resume effect queue processing. For example, an effect might trigger a modal to be dismissed by user interaction. When this effect fired, the modal could request that effect playback be paused and then resume playback when closed.

This also solves for client-determined dynamic effect durations. A client could pause/resume to enforce its own timing instead of relying on the timing set in the plug-in.