[added] ability to change default 'ReactModalPortal' class

[manuhabitela]

Changes proposed:

• new portalClassName prop to change the ReactModalPortal class.

Upgrade Path (for changed or removed APIs):

Acceptance Checklist:

• All commits have been squashed to one.
• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

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Thanks for this project!

What do you think about adding this?

I think this is useful because:

• code can be cleaner when also using custom content and overlay classnames. If using BEM classes, it makes more sense to have a DOM like .CustomModal > (.CustomModal-content + .CustomModal-overlay) and not like .ReactModalPortal > (.CustomModal-content + .CustomModal-overlay).
• my current use case is opening a modal from a content-script in an browser extension. I want to be able to target the portal DOM element precisely in order to prevent conflicts with the page css.

Few questions on my side:

• I went with "portalClassName" but I'm not sure if this is really speaking to everyone.
• I added a brief sentence about the change in the README but didn't add anything in the examples. I figured the readme change would be enough, but let me know if you don't think so.

[claydiffrient]

Looks good to me and I see no reason this can't go through.