Feature: meta selectably emits once per write

Background: I'm writing an app that pulls data from RSS streams and writes it into a FeedParser, one whole feed at a time.

Problem: the ```meta``` event only fires once per FeedParser, meaning that no matter how many RSS feeds I shovel into a FeedReader, I only ever get one ```meta``` event.

Solution: this pull request implements a new option, ```emit_meta_once```. By default it's set to true (the current behavior). If you set this to ```false```, ```this._emitted_meta``` gets cleared on every ```_write``` so that downstream consumers can see a ```meta``` event for each feed XML string/buffer they shovel in.

I modified the tests to check the new behavior; all passing.