Add support for appending modal node to non-root nodes

[ajclaasen]

Heya, thanks for this package, it's saved me & my team quite a bit of time already & it works great!

I am running into an issue when testing components that use the modals from this package. Because this package appends modals to the document root, and not to the element calling the render, it is causing them to persist between tests.

I am using react-testing-library, which, as part of its optimalisations, does not rerender the entire document between specs, but only rerenders the component being tested¹.

Looking at the source code of react-confirm, it seems that it should not be too hard to add support for appending the modal elsewhere² by passing the node to be appended to to the createConfirmation method.

Footnotes

1. https://github.com/testing-library/react-testing-library/blob/ccd8a0d97dd9da0a420f2cf012a24d414d1646ed/src/pure.js#L208 ↩

2. https://github.com/haradakunihiko/react-confirm/blob/3f6a23c353338f7e0302b6e6c5e53c8d46c3e899/src/createConfirmation.js#L4-L6 ↩