Added support for objects as properties

[OCA99]

With this change, any object that defines both get() and set() methods that return and accept an animatable value (number or string) can be used as a property for a target.

This is useful as one may want to use custom setters and getters along with the anime library. In my case, I am using a custom reactive library in which observable values provide custom setters and getters in order to emit update events to observers in each animation step.

[juliangarnier]

Hey thank you for the PR, but this is too specific to a single (and rarely used) use case to be incorporated into the core.

Why not use https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get instead ?