_findHost in included for third addons

[betocantu93]

In order to let other ui libs such as ember-paper use ember-flatpickr via third addons integrations like ember-paper-flatpickr the host app via ember-cli-build.js should be able to define flatpickr options, like the theme or locales, as normal ember-flatpickr do... so this PR makes sure we find the host app, so we can access the app.options.flatpickr options.

It's actually a copy paste from ember-paper include hook.

[RobbieTheWagner]

@betocantu93 I assume you tested this and it works for your addon?

[betocantu93]

@rwwagner90 yes, I tested it in my addon and it works