Link to the official docs by default

Since this addon is Ember specific, does it make sense to link to the official ember & ember data yuidoc when generating a blueprint? Like,

```
"external": {
"data": [
{
"base": "http://emberjs.com/api/",
"json": "http://builds.emberjs.com/tags/v2.7.1/ember-docs.json"
},
{
"base": "http://emberjs.com/api/",
"json": "http://builds.emberjs.com/tags/v2.7.0/ember-data-docs.json"
}
]
}
```

Start by locating the blueprint generation entry point in this Ember CLI addon and inspect how the generated external documentation data is configured. Use the Ember and Ember Data YUIDoc URLs shown in the issue as the expected defaults, then verify that a generated blueprint includes both links.

Written by the indexing model from the issue text.