Allow flatpickrRef access via an action

[cah-brian-gantzler]

Currently access to the flatpickrRef is only done via two-way binding which is against DDAU. This also prevents the component being used with angle-bracket syntax.

Created an action onFlatpickrRef to replace the two-way binding.

[RobbieTheWagner]

Thanks for the PR @cah-briangantzler! My one concern here is the naming. Perhaps setFlatpickrRef instead of onFlatpickrRef?

[cah-brian-gantzler]

Most actions usually are preceded by on, but I can change to set if you wish. setFlatpickrRef though would seem to be you are setting something on the ember-flatpickr component when in fact the action will setting something on the parent. How about getFlatpickrRef or onGetFlatpickrRef?

[RobbieTheWagner]

getFlatpickrRef works for me. on implies something is changing and I've typically seen things like onChange onUpdate etc. I want to make it clear it's for getting access to the flatpickr reference, and not something to observe changes for when the flatpickr reference is set.

[cah-brian-gantzler]

I have updated this