Adjust generated scenarios

As mentioned in https://github.com/ember-cli/ember-try-config/pull/3 there are some situations where it would be useful to adjust the list of generated scenarios afterwards by e.g. adding or overwriting certain dependencies or even cloning generated scenarios.

Instead of implicitly generating the scenarios when `useVersionCompatibility` is used I would suggest to explicitly `require('ember-try-config')` and then put something like `scenarios: generateScenarios()` in your `config/ember-try.js` file.

That way the `generateScenarios()` result could be written into a temporary variable if needed and then adjusted afterwards.

/cc @rwjblue @kategengler