Report specificity mode

[bartveneman]

As @csswizardry writes in his article https://csswizardry.com/2016/06/improving-your-css-with-parker/, it would be nice to be able to the specificity that is most common in your stylesheet.

It would be quite nice to know that ‘most of your selectors have class-level specificity’, rather than ‘the average specificity across all of your selectors is roughly that of a class’. A subtle but significant distinction.

stylesheets: {
  specificity: {
    mode: 
      total: 1234,
      share: 0.32,
      specificity: {
        a: 0,
        b: 0,
        c: 2,
        d: 0
      }
    }
  },
  top: [{ ... }}
}

[bartveneman]

Ugh, dupes #58