HLS-25: Additional flags/features to aid Low Latency Streaming

I've been doing some tinkering with creating Ultra-low Latency HLS streams, which seems to have gone quite well (largely comparable to RTMP, depending on the player's settings).

However, there are a few things I needed to hack in (or plain skip) that would be nice to have as options to HLS-Stream-Creator (especially as some are applicable to standard latency streams too)

Raising this issue to act as a parent to various subtasks