Jack Steinberg

Currently the `runTest` function in [testUtils.js](https://github.com/facebook/jscodeshift/blob/master/src/testUtils.js#L59) hardcodes `.input.js` and `.output.js` to the test filenames it looks for, which doesn't allow for running test files with `.mjs` extensions ([ES modules](https://tc39.es/ecma262/#sec-modules)). Can...