Remove Modal.setAppElement?

[claydiffrient]

I'm thinking perhaps we should get rid of Modal.setAppElement. It's slightly odd and trips many people up when trying to use the library. There's also been the odd appElement prop that has been around.

I think we should remove the appElement prop replacing it with a getAppElement function prop similar to what was done for the parentElement stuff in 1.6.x. I'm almost certain this would help with things like #174 is having issues with.

Thoughts?

[diasbruno]

Maybe, an optional attribute if someone needs to set a different element for a particular modal.
Default it to document.body would be better (common case).