Support Azure app configuration strings

Since we don't want to check in config.js and its config strings and we
want continuous deployment to Azure, use the Azure management portal's
App Settings key / value strings to specify our private credentials.

Use the `optional` node module to make config.js optional and fill in
its strings with environment variables.  This should be refactored into
a separate config module that handles the fakery so that index.js
doesn't need to know about it.  That'll get done when I need config data
in multiple places at once.