Fix the animation component for custom vec3 properties

[Galadirith]

Description:

Fix the animation component to correctly animate vec3 properties of custom components. The current implementation handles the cases for position, scale and rotation but does not handle custom properties that do not represent properties of Object3D (PROP_POSITION, PROP_ROTATION, PROP_SCALE).

Changes proposed:

• Change the animations property reference used for custom vec3 properties
  

  aframe/src/components/animation.js

  Line 404 in ff2c7a4

  var value = anim.animations[0].target;

  
  to animatables as is used for position, scale and rotation
  

  aframe/src/components/animation.js

  Line 377 in ff2c7a4

  var value = anim.animatables[0].target;

My thanks:

You all really inspire me ❤️ I am so grateful for all the hard work you've all put into developing A-Frame, and your commitment to an open VR web (sorry for being unprofessional and including this message in an PR).

[ngokevin]

Thanks much for the kind words, and the solid PR and tests!