Support multiple modules.

Ember is migrating from `import isEnabled from 'ember-metal/features'` to `import { isFeatureEnabled } from 'ember-metal';`. During the transition (while the code is being updated piece by piece) it would be nice to instruct this plugin to simply process both modules. Unfortunately, that doesn't seem possible today.

One possible solution is something like:

``` js
{
"plugins": [["feature-flags", [
{
"import": {
"module": "ember-metal/features"
},
"features": {
"new-feature": "disabled"
}
},
{
"import": {
"module": "ember-metal",
"name": "isFeatureEnabled"
},
"features": {
"new-feature": "disabled"
}
}
]]
}
```

@mmun - Thoughts?