Add ability to set deployment config settings via environment

[techieshark]

Fixes #104 (adds ability to put people.js in Mesa's own repo).

Also adds ability to specify a number of other config settings via the environment (.env for dev/test, heroku config command or 'Deploy to Heroku' button for production).

Settings at this time include:

• Google Analytics ID
• Mapbox ID, map center, map marker location
• people.js location
• Disqus settings
• URL for ESRI API that returns council districts