feat(lib-transformer): specify event type inside of chunk of streamed…

[zjakin]

Allows sending a stream of events of different types through a single channel.
How to use:

On server side

    stream.write({ event: 'SPECIFIC_EVENT', 'some data' }); 
    stream.write({ event: 'OTHER_EVENT', 'some data' }); 
    reply(stream);

On client side

eventSource.addEventListener('SPECIFIC_EVENT', (e) => {
                const results = JSON.parse(e.data);
                this.hosts = results;
            }, false);
eventSource.addEventListener('OTHER_EVENT', (e) => {
                const results = JSON.parse(e.data);
                this.hosts = results;
            }, false);