Allow a different initial interval in Writer

For [StreamMachine](https://github.com/StreamMachine/StreamMachine), I'm working on being able to hand off all connections to a new process to allow seamless restarts.  To do that with an Icecast writer, I need a way to tell the new process that it should write less than a full chunk of data before the first meta injection.

I'm grabbing `_parserBytesLeft` from the original icecast writer, and then passing that in to the new one as `initialMetaInt`.