Support Events outside of HTML spec

[tot0]

Make EventBuilder a trait, with a default SpecCompliantEventBuilder.

Allows for custom EventBuilders to handle RawEventLines which aren't apart of the HTML SSE event spec. Not really the cleanest way to do this, but it's what I could think of doing now, would need to be a breaking change if upstreamed :(

[GlenDC]

Can you name some industry examples @tot0 on why this non-spec compliant thing is ever used? And outside the web don't you have much better options anyhow than SSE which anyhow isn't that good to begin with?