Schematics: Add schematics for action creators

[brandonroberts]

Follow up to #1480

1. Adds an actionCreators option to the action, feature, entity schematics that uses the new action creator functions instead of classes
2. The actionCreators flag will use a different set of files for scaffolding. This keeps our current structure with files intact.
3. Adds tests to verify the behavior of the previously mentioned schematics

[timdeschryver]

What about making actionCreators the default (and make it our recommended way)?
We already have refactored the example app and are currently busy updating the docs.

[brandonroberts]

I'd rather it be opt-in for 8 and the default for 9. We'll see how the betas go and revisit.

[jtcrowson]

Happy to pick this up if it's still up for grabs.

[timdeschryver]

If I'm not mistaken, I think @brandonroberts has started on this.

[brandonroberts]

Yep, I have a branch here. https://github.com/ngrx/platform/tree/schematics-action-creators

If you want to take it, that's fine with me. It will need to land after #1725