Schematics: Generate selectors file as part of feature schematic

[brandonroberts]

It's common practice to have a separate {feature-name}.selectors.ts file alongside your actions, reducers, etc. The feature schematic would add a flag to generate this file and it's tests in addition to the current set, and eventually become the default.

We could optionally do this for the entity schematic also.

At a minimum, the selectors file would:

• Import the usual createSelector functions
• Import the reducer exports
• Include a selector that uses createFeatureSelector to select that feature key

[timdeschryver]

If no-one is currently working on this or is planning to work on this feature, I could grab this.