Allow client interface to be turned on/off with ENV vars

Building on pull request https://github.com/remy/jsbin/pull/610, this allows these settings to be configured as an Environment variable.  This is needed for Heroku support, see pull request https://github.com/remy/jsbin/pull/607
Merge pull request #614 from ably-forks/interface-config-from-env-vars

Allow client interface to be turned on/off with ENV vars