Initialize hideHighlight in Interactable.cs

[tapiralec]

Fixes issue where creating Interactables at runtime by using AddComponent<Valve.VR.InteractionSystem.Throwable>() on a GameObject would throw null reference exception due to hideHighlight not being initialized, preventing itself and its children from showing highlighting.

[zite]

Thanks for the catch, and the PR. This will be in the next release.