Use PostCSS

# Upgrade to use PostCSS, which is pretty rad.

## This has a few breaking changes, so will be a major version bump.
- `declarations.byProperty`, `declarations.unique`, and `declarations.byMedia` are now arrays of declaration objects, whereas before they were number arrays that referenced the `declarations.all` array. This should make working with the data a little easier, but the cssstats.com app controller will need to have the `parseDeclarations` function removed.
- Properties in the aggregates object are now singular instead of plural. This is because it takes the property and camelCases it automatically.

## New
- The properties in the aggregates object now have all properties used in a stylesheet.
- Base 10 specificity score is now included in the selectors array, so the app's `parseSpecificity` function can be simplified.
- Gzip stylesheet size using the `gzip-size` module.
- Rename the npm package to `cssstats` (hopefully this doesn't break anything).

cc @mrmrs