Add shouldComponentUpdate-ish feature

[filipbech]

this will make things faster and we can make immutable data a native citizen

[kenchris]

This is not needed if my PR gets merged in lit-html lit/lit#239

[filipbech]

Im not sure that eliminates the need completely... it does cover the most important part though...

What if a property change causes something async to happen (a fetch or similar) that then needs to invalidate... being able to opt out of re-rendering could still be a nice feature...

or what I know my data is always immutable - I can skip the object equality check altogether...