Support import and export of ES2015 modules (#4300)

This pull request adds support for ES2015 modules, by recognizing `import` and `export` statements. The following syntaxes are supported, based on the MDN [import](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import) and [export](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export) pages:

```js
import "module-name"
import defaultMember from "module-name"
import * as name from "module-name"
import { } from "module-name"
import { member } from "module-name"
import { member as alias } from "module-name"
import { member1, member2 as alias2, … } from "module-name"
import defaultMember, * as name from "module-name"
import defaultMember, { … } from "module-name"

export default expression
export class name
export { }
export { name }
export { name as exportedName }
export { name as default }
export { name1, name2 as exportedName2, name3 as default, … }

export * from "module-name"
export { … } from "module-name"
```

As a subsitute for ECMAScript’s `export var name = …` and `export function name {}`, CoffeeScript also supports:
```js
export name = …
```

CoffeeScript also supports optional commas within `{ … }`.

This PR converts the supported `import` and `export` statements into ES2015 `import` and `export` statements; it **does not resolve the modules**. So any CoffeeScript with `import` or `export` statements will be output as ES2015, and will need to be transpiled by another tool such as Babel before it can be used in a browser. We will need to add a warning to the documentation explaining this.

This should be fully backwards-compatible, as `import` and `export` were previously reserved keywords. No flags are used.

There are extensive tests included, though because no current JavaScript runtime supports `import` or `export`, the tests compare strings of what the compiled CoffeeScript output is against what the expected ES2015 should be. I also conducted two more elaborate tests:

* I forked the [ember-piqu](https://github.com/pauc/piqu-ember) project, which was an Ember CLI app that used ember-cli-coffeescript and [ember-cli-coffees6](https://github.com/alexspeller/ember-cli-coffees6) (which adds “support” for `import`/`export` by wrapping such statements in backticks before passing the result to the CoffeeScript compiler). I removed `ember-cli-coffees6` and replaced the CoffeeScript compiler used in the build chain with this code, and the app built without errors. [Demo here.](https://github.com/GeoffreyBooth/coffeescript-modules-test-piqu)
* I also forked the [CoffeeScript version of Meteor’s Todos example app](https://github.com/meteor/todos/tree/coffeescript), and replaced all of its `require` statements with the `import` and `export` statements from the original ES2015 version of the app on its `master` branch. I then updated the `coffeescript` Meteor package in the app to use this new code, and again the app builds without errors. [Demo here.](https://github.com/GeoffreyBooth/coffeescript-modules-test-meteor-todos)

The discussion history for this work started [here](#4160) and continued [here](GeoffreyBooth#2). @lydell provided guidance, and @JimPanic and @rattrayalex contributed essential code.