Allow appElement prop to call setAppElement

Summary:

Currently I don’t think react-modal is compatible with code splitting using loadable-components because of needing to call Modal.setAppElement(). The issue is that if we load the Modal element like this:

const Modal = loadable(() => import('react-modal'))

…then Modal won’t have resolved at runtime when we call Modal.setAppElement.

My suggestion is to call setAppElement from within the constructor if we pass an appElement prop to the Modal.

Thanks for maintaining this repo, and please let me know if a PR to do the above would be welcomed, as I’d love to contribute!

[YPCrumble]

Thanks again! The code works as you wrote it above 😄.

[YPCrumble]

@diasbruno so sorry - your code works I had just implemented it wrong! Thanks again for this library and your patience with my issue - closing.