Introduce whitelisted level 1 optimizations

[jakubpawlowicz]

As mentioned it briefly in #842 the end goal of level 1 optimizations is to operate in whitelist-mode not blacklist-mode as it's done now.

We currently have a compactable object defining how properties behave in level-2 optimizations. The plan is to make it more universal so it can also hold info on how to optimize properties, e.g.

var compactable = {
  'width': {
    canOverride: canOverride.generic.unit,
    defaultValue: 'auto',
    optimize: [
      optimize.pixelLengths,
      optimize.precision,
      optimize.units,
      optimize.zeroUnits
    ],
    shortestValue: '0'
  }
};

This way it'd be easier to:

• understand what's going on in level 1 optimizations;
• add extra optimizations on per-property basis;
• pinpoint issues if anything goes wrong as currently all optimizations are applied to each and every property;
• write optimization code;

There are some drawbacks too:

• it won't be easy to optimize multi-value or shorthand properties, e.g. text-shadow or background respectively, without breaking them down to components, optimizing, and then restoring, which is currently a part of level 2 optimizations;
• it's gonna be verbose but I actually prefer less magic.