Add option to specify the "runner" host - also fixes regexp bug in live.js

I updated jsbin to allow the user to specify a "runner" host in config.*.json

Currently, jsbin will replace "jsbin" with "run.jsbin" in the URL when creating the iframe for the runner. This causes a problem if, say, someone uses jsbin.example.com as their url and doesn't have a wildcard DNS entry to ensure that run.jsbin.example.com works :)

Thoughts? Did I make a big huge mistake anywhere? It's my first NodeJS PR.