* [react-is] Initial typings for package
React introduced the react-is for assertions on elements and element types.
I took the README and tests and created the matching ts types.
* Add TypeScript 2.6 comment
this package uses @types/react, and it requires 2.6 or above.
* Match strictFunctionTypes config of react
* Remove unneeded generics
per https://github.com/Microsoft/dtslint/blob/master/docs/no-unnecessary-generics.md
React uses this pattern (providing the Props interface via generics) and turned off the linting. We'll use the default lint options and `any` props.
* Use ReactType instead of an explicit union
* Cleanup unused imports
