Add support for variables on an environment

[duncan3dc]

At the moment we allow variables defined globally, and variables defined on a resource. We should probably allow variables to be defined on an environment too (so that users aren't forced to use the perEnvironment function and move this value to the template instead of the config file

environments:
  - name: "production"
    vars:
      DebugMode: 1
  - name: "nonprod"
    vars:
      DebugMode: 0

resources:

  - name: "myapp"
    template: "app"
    vars:
      Program: "myapp"

See #28 for original implementation