ember modifier v4

[jkusa]

resolves #390

[miguelcobain]

Ideally, we would use a macro like this to support both versions of ember-modifiers: https://github.com/DockYard/ember-in-viewport/blob/master/addon/modifiers/in-viewport.js#L14

[MelSumner]

This would resolve the deprecation message in the console when using the current version of the addon, yes?

(creating a function-based modifier with { eager: true } is deprecated and will be removed at v4.0 [deprecation id: ember-modifier.function-based-options] This will be removed in ember-modifier 4.0.0.)