React-dropzone: Add support for maxFiles

[depiction]

• [ ] I found a bug
• [X] I want to propose a feature

What is the current behavior?

Users may add as many files to a dropzone as they want. There is a multiple prop, but that only prevents users from selecting multiple files at a time.

What is the expected behavior?

Add a maxFiles prop that accepts a number, allowing users to only add as many files to the dropzone area as allowed. Any files above the limit will be rejected.

If this is a feature request, what is motivation or use case for changing the behavior?

This is a requirement in my current project and I think it's a fairly common need.

[rolandjitsu]

You're welcome to make a PR as I don't have time to work on it.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity in the past 60 days. It will be closed if no further activity occurs. If the problem persists please comment here to bump your issue. Thank You - React Dropzone Maintaners