Using custom endpoint URL?

[chenxiaoqino]

As suggested on certstream website, non-experimental use should not use the public server and should setup separate certstream server. However, currently the URL wss://certstream.calidog.io is hard-coded into the library.

I suggest adding the URL endpoint parameter for CertStreamEventStream. I can contribute the code and update the example if needed.

[pathtofile]

Added PR for this: #11