Add ability to specify Modal parent

Issue description

• components: Modal
• reactstrap version 5.0.0-beta
• react version 16.3.0
• bootstrap version 4.0.0

What is happening?

There is currently no way to specify a Modal container. All Modals appear as children of the body.

What should be happening?

Modal should provide a parentSelector prop, similar to the one in the react-modal library.

See: http://reactcommunity.org/react-modal/#usage

/*
  Function that will be called to get the parent element that the modal will be attached to.
*/
parentSelector={() => document.body}

This should be a relatively minor feature. I’d be happy to implement and create a pull request myself (although that’s not necessary.)

I’m creating this issue mostly to test the water and get feedback from the reactstrap devs.

[supergibbs]

@leebradley That is a cool use case but can it be done with bootstrap CSS? The goal of reactstrap is to make bootstrap easy to use with react by creating components that apply the correct class names and replacing the jquery library with react. Since bootstrap only supports full page modals, this is out of scope for reactstrap.

@tannerhallman Hopefully soon! @TheSharpieOne manages the releases.

[ameliabradley]

@supergibbs That’s a good point. It’s not supported in vanilla Bootstrap 4. I didn’t realize that Bootstrap fixed-positioned the modal element by default, but I guess that makes sense.