Add debug tooling babel plugins.

[rwjblue]

Implements ember-cli/rfcs#50 and the @ember/debug portion of emberjs/rfcs#176.

Below is the documentation being added to the README explaining the feature.

---

Debug Tooling

In order to allow addons to easily provide good development mode ergonomics (assertions, deprecations, etc) but still perform well in production mode ember-cli-babel automatically manages stripping / removing certain debug statements. This concept was originally proposed in ember-cli/rfcs#50, but has been slightly modified during implementation (after researching what works well and what does not).

Debug Macros

To add convienient deprecations and assertions, consumers (apps and/or addons) can do the following:

import { deprecate, assert } from '@ember/debug';

export default Ember.Component.extend({
  init() {
    this._super(...arguments);
    deprecate(
      'Passing a string value or the `sauce` parameter is deprecated, please pass an instance of Sauce instead',
      false,
      { until: '1.0.0', id: 'some-addon-sauce' }
    );
    assert('You must provide sauce for x-awesome.', this.sauce);
  }
})

In testing and development environments those statements will be executed (and assert or deprecate as appropriate), but
in production builds they will be inert (and stripped during minification).

General Purpose Env Flags

In some cases you may have the need to do things in debug builds that isn't related to asserts/deprecations/etc. For
example, you may expose certain API's for debugging only. You can do that via the DEBUG environment flag:

import { DEBUG } from '@glimmer/env';

const Component = Ember.Component.extend();

if (DEBUG) {
  Component.reopen({
    specialMethodForDebugging() {
      // do things ;)
    }
  });
}

In testing and development environments DEBUG will be replaced by the boolean literal true, and in production builds it will be replaced by false. When ran through a minifier (with dead code elimination) the entire section will be stripped.

Disabling Debug Tooling Support

If for some reason you need to disable this debug tooling, you can opt-out via configuration.

In an app that would look like:

// ember-cli-build.js
module.exports = function(defaults) {
  let app = new EmberApp(defaults, {
    'ember-cli-babel': {
      disableDebugTooling: true
    }
  });

  return app.toTree();
}

In an addon that would look like:

// index.js

// ...snip...
init() {
  this._super.init.apply(this, arguments);

  this.options['ember-cli-babel'] = this.options['ember-cli-babel'] || {};
  this.options['ember-cli-babel'].disableDebugTooling = true;
}
// ...snip...

[cibernox]

LGTM

[stefanpenner]

This looks awesome (but i did leave some nit-picks)

[stefanpenner]

addons and apps?

[stefanpenner]

Is it worth documenting how an addon would do this? This seems quite off the beaten path

[rwjblue]

Pushed an update for the two README tweaks that @stefanpenner mentioned...

[stefanpenner]

can you add a bulleted list of available macros, or a link to where they are documented?

[rwjblue]

Good point, updated!

[stefanpenner]