Add support for es6 imports

[ConnerAiken]

It would be nice to support CommonJS/ES6 imports for the ObservableSlim and Nimbly libraries for transpiling/minification/bundling purposes.

Boilerplate repo: https://github.com/ConnerAiken/nimbly-boilerplate

The master branch is a working boilerplate.
The es6-import branch is an example of how I would expect the library to be imported using es6 modules.
The es5-import branch is an attempt at using CommonJS (require) for the libraries, but I get the error: MutationObserver is not a constructor

[ElliotNB]

Looked into this with Conner today -- does support ES6 imports but the dependency injection wrapper is required. Closing for now, but we probably should modify the Nimbly examples to show a working import.