Add a polymorphism for autoRescale settings.

[Yomguithereal]

Would be nice to be able to determine autoRescale option for only nodes and edges' sizes but not position for instance.

The proposition here is to create a polymorphism on the autoRescale option:

// Enable full autoRescale
settings.autoRescale = true;

// Fully disable autoRescale
settings.autoRescale = false;

// "à la carte" autoRescale
settings.autoRescale = ['nodes.size', 'edges.size'];
settings.autoRescale = ['nodes.position'];

@sheymann, is that helping for #338 ?

[sheymann]

Yes it would definitely make sense.