Add support for using factory-guy in other environments

[juwara0]

Closes: #254

CHANGELOG:
Updated index.js to support using factory-guy in other environments
Updated README to show configuration needed for using factory-guy in other environments

DESCRIPTION:
This PR adds support for using factory-guy in other environments to the base cases of development and test.

The user just needs to add:

// file: config/environment.js
ENV.factoryGuy = true;

to the environments in which factory-guy is desired.

By default, the test environment is still enabled but now a user can turn it off by specifying:

// file: config/environment.js
if (environment === 'test') {
  ENV.factoryGuy = false;
  ...
}

[danielspaniel]

Looks good Michael
I was just about to ask how this was going. Ok .. one question.
The only caveat is to make sure in test environment to not put => factoryGuy:true .. or else it would look for the scenarios/main.js file and barf if not found.

[juwara0]

@danielspaniel - I have added a note to the README to highlight that factoryGuy: true should not be added to the test environment.

[danielspaniel]

https://github.com/danielspaniel/ember-data-factory-guy/releases/tag/v2.8.2 has this PR included