Add the option to pass in a :url to the repl-env function

I need this option, as I interact with my development environment through a VMWare Linux guest. So I need flexibility beyond the hard-coded "***localhost***" endpoint. 

Using the [demo app](https://github.com/cemerick/austin/tree/master/browser-connected-repl-sample), I tested this locally. In the cljs repl, I got one "***java.io.IOException: Broken pipe***". But even then, the browser repl always worked.