feat: adds removeFiles, onRemoveFiles

[estrattonbailey]

What kind of change does this PR introduce?

• bugfix
• feature
• refactoring / style
• build / chore
• documentation

Did you add tests for your changes?

• Yes, my code is well tested
• Not relevant

If relevant, did you update the documentation?

• Yes, I've updated the documentation
• Not relevant

Summary
I came across a situation where I'd like to be able to remove files from the internal state of react-dropzone. I saw that others have asked for this, and that help was needed.

This PR introduces two new API methods onRemoveFiles and removeFiles:

<Dropzone onRemoveFiles={files => console.log('removed', files)}>
  {({ removeFiles, acceptedFiles }) => (
    <button onClick={() => removeFiles(acceptedFiles[0])}>Remove one file</button>
  )}
</Dropzone>

onRemoveFiles will always receive an array of Files, and removeFiles always accepts an array of Files. This makes it easy to remove multiple files at a time, say to "clear" the dropzone if needed.

Does this PR introduce a breaking change?
Shouldn't.

Other information
I could use some feedback on the tests, types, and JSDoc. Generally tried to stick to the patterns in place, but lmk if those need work.

Also, thanks for the library :)

[nylon22]

Array.prototype.find is not supported in IE: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/find

This project doesn't have an official list of supported browsers. See #630. That being said, the project isn't using find anywhere else, so this would be a breaking change for any project that isn't polyfilling find and that has IE users.

I don't maintain this project but I think it would be better to use Array.prototype.some instead. :)

[estrattonbailey]

@nylon22 thanks for the feedback.

In interest of time, I've actually just wrapped this library to create an API that's more suitable to my needs. I made a gist that outlines the approach I took.

Because of that, I'm not going to follow up on this PR, and I think I'll close to help reduce clutter.

[Angelk90]

@estrattonbailey : I have a similar problem can I ask you for a hand on something?

[rajesh-cp]