Importing emberx-file-input in an addon

[xcambar]

Unfortunately, #28 reverts #26 (aka, remove ember-cli-import-polyfill, mostly) which allowed the addon to be imported from another addon.

It breaks an important use case for us, as we import this addon from other (private) addons. I'm taking the opportunity to start the discussion here about what should be the best approach, considering that ignoring this use case was intentional.

FYI, the polyfill is meant for an upcoming feature in Ember 2.7 that allows nested addons. See ember-cli/ember-cli#6043 for the implementation.

ping @Robdel12 @cowboyd

[Robdel12]

Woops, at first I thought it went out with a release. Thankfully it didn't :)

Thanks for bringing this to my attention (again), I opened a PR and once those tests pass I'm going to merge it. 🎉