Add className to containing div

[devinmcinnis]

Since this has been referenced from multiple issues, I want to add that even though react-portal does not allow setting a class name, it would be beneficial to have a class name for the component.

Currently, there is no way to target the containing <div> wrapper.

My use case is supporting a modal on iOS Safari. (CSS) Viewport units aren't functional/reliable with the expanding/collapsing menus and without being able to add height: 100% to the react-portal <div>, it's impossible to properly support.

#105 (comment)

[tajo]

Hey, please check the new major version (complete rewrite) of react-portal: #157

It's React v16 only since its uses the new official Portal API. There is the first beta released and I would like to get your feedback. I don't have bandwidth to maintain v3 which is very different and full of hacks.

If you feel your PR still applies, please rebase it against the master and re-open it. Sorry for the lack of response in past! Thanks!