Use client-hostname by default

[SevereOverfl0w]

This is more likely to be correct, that you want to connect back to the same hostname you connected to the application on.

The binding hostname that was used otherwise is more likely to be an internal IP such as 0.0.0.0 which is not a valid way to access the web anymore.

Fix #351

[bhauman]

Thanks for the PR! The 0.0.0.0 problem is real and worth fixing, but I'm not sure [[client-hostname]] is the right default here.

[[client-hostname]] resolves to js/location.hostname on the client. This breaks in a couple of scenarios:

1. Sideloaded REPL — if figwheel's JS is loaded into a page served from a different server (e.g. your app on myapp.dev:3000, figwheel on localhost:9500), js/location.hostname gives you myapp.dev, not localhost, and the websocket connection fails.

2. Node.js / React Native / Web Workers — fill-url-template in figwheel-repl only resolves [[client-hostname]] when host-env is :html. For Node etc., the placeholder passes through as a literal string and the connection fails.

I think the narrower fix is to keep [[config-hostname]] as the default but translate 0.0.0.0 to localhost server-side in fill-connect-url-template, since 0.0.0.0 is a valid bind address but never a valid connect address.