Add a Babel step to transpile to ES2017

[djcsdy]

At the moment all code compiled with webpack-config has to be valid ES5, including third-party libraries from npm. Any code that is invalid ES5 will fail at the uglify step, and produce an error message that makes the problem very difficult to track down.

This is becoming increasingly untenable. We should insert Babel into the build process to transpile ES6+ code down to ES5.

See also #70.

[djcsdy]

We now target ES2017 but we should still consider adding a Babel step to compile down from ES2018+.