SSE (Server Sent Events) support

[jacorall]

It will be nice to have SSE support like the susie addon (https://github.com/mtharrison/susie) in HAPI. One will then be able to push messages from the server.

[sholladay]

Sure, I'm open to it. I would recommend trying a userland implementation first and adding whatever APIs to Pogo that are necessary to support that, but I would also accept a PR adding it directly to core.