Allow :ref as a component attribute

[kaosko]

Current reagent allows :ref attribute for referring to the component itself at render time. For re-com components, using :attr {:ref #my-function} works but it's flagged as a warning: "Validation failed for argument ':attr' in component ...". Would certainly be easy to add :ref to the attribute validation, but perhaps :ref should be supported as a first-class attribute across any re-com component?

[Frozenlock]

I have dozens of warnings because of this. I think it should remain inside :attr.

[Frozenlock]

For the validation, I propose to add a section react with those 2 keys:

• :ref
• :key

( https://github.com/day8/re-com/blob/master/src/re_com/validate.cljs#L181)