Keep arrays as arrays

Hi guys! First of all, great work on this project!

I was thinking if you find it useful to keep arrays as arrays on the resulting diff object, something like:

      let difference = diff(l[key], r[key])
      if (Array.isArray(l[key])) {
        difference = Object.entries(difference).map(d => d[1])
      }

If it's interesting to you I can submit a PR to do that!

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