Support for importing CommonJS style modules (like rbush)

This extends @tsauerwein's work in #2863 to provide a general way to import CommonJS modules.  By adding a dependency to our `package.json` and adding the module name to the `ext` array, we get a `goog.provide` that can be `goog.require`d from other parts of the library.

For example, the `package.json` snippet below would allow us to `goog.require('ol.ext.foo')`:
```json
{
  "dependencies": {
    "foo": "1.0.0"
  },
  "ext": ["foo"]
}
```

This assumes that the `foo` package includes a browser-ready `main` script (specified in its `package.json`) that assigns a function to `module.exports`.  Additional work may be needed to support modules that assign non-function values to `exports`.