React 17 support

Hi,

When I try to install this package with `react@17.0.1` (latest at time of writing), it fails with a dependency error.

Is there any workaround, or plans to support React 17?

## Steps to reproduce

```
mkdir tempproj
cd tempproj
npm i react@latest
npm i react-outside-click-handler@latest
```

## Error

```
npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! Found: react@17.0.1
npm ERR! node_modules/react
npm ERR! react@"^17.0.1" from the root project
npm ERR! peer react@"^0.14 || >=15" from react-outside-click-handler@1.3.0
npm ERR! node_modules/react-outside-click-handler
npm ERR! react-outside-click-handler@"1.3.0" from the root project
npm ERR! 1 more (react-dom)
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer react@"^0.14 || ^15.0.0 || ^16.0.0-alpha" from airbnb-prop-types@2.16.0
npm ERR! node_modules/react-outside-click-handler/node_modules/airbnb-prop-types
npm ERR! airbnb-prop-types@"^2.15.0" from react-outside-click-handler@1.3.0
npm ERR! node_modules/react-outside-click-handler
npm ERR! react-outside-click-handler@"1.3.0" from the root project
npm ERR!
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force, or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.
```

## Context

It looks like this is caused by the dependency on `airbnb-prop-types`, which currently has an open ticket for React 17 support: https://github.com/airbnb/prop-types/issues/73