fix: Check drag event contains files before showing "copy" cursor

closes react-dropzone#1042

fix: allow validator to be null

feat: add {validator} for custom validation

fix: add supported browsers via browserslist config and close react-d…

…ropzone#630

fix: update internal state when maxFiles prop changes and close react…

…-dropzone#1025

fix: set isDragReject true when maxFiles rejects (react-dropzone#1020)

fix(deps): update file-selector and close react-dropzone#1022

feat: add {maxFiles} to limit the accepted files

feat: improve Typescript support for errors

fix: define functions as properties

This is more correct for TS strict mode.
See info here: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/method-signature-style.md

It also is compatible with the unbound-method rule.