Allow server to push content to the client

[Kaliumhexacyanoferrat]

As a developer of client application processing server events, I would like to be able to easily stream data to the client, so that I can process the data on the client-side.

Example

[ResourceMethod(path: "test")]
public IHandlerBuilder Test(IRequest request)
{
    return Push.Create(async (stream) => {
         await steam.WriteAsync(...);    
    });
}

Acceptance criteria

• The functionality is implemented in the IO module

Notes

See #309, #371.

Specc, see https://html.spec.whatwg.org/multipage/server-sent-events.html#server-sent-events

[Fiahblade]

When I find the time I might take a look at this, no promises ;)