Compare multi-benchmarks.

**Issue by [BrianHicks](https://github.com/BrianHicks)**
_Monday Feb 13, 2017 at 16:11 GMT_
_Originally opened as https://github.com/BrianHicks/elm-benchmark/issues/6_

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Right now `compare` only compares two single benchmarks (that is, `Benchmark`s created with `benchmark` through `benchmark8`.) We need to be able to compare any valid `Benchmark`. I suggest the following:

- for two `Group`s (that is, `Benchmark`s created with `describe`) we will zip the elements using `compare`, and modify the inner names. If one list is longer than the other, we will only zip the common elements and drop the rest. This will return a `Group` named "{group a} vs {group b}" whose elements are `Compare`s named "{element a name} / {element b name}" with children named "{group a}" and "{group b}". This logic recurses if any child elements are groups themselves.
- for two `Compare`s (that is, `Benchmark`s created with `compare`) we will combine as if they're groups with exactly two elements each.
- for mixed elements, we will not combine or change structure, and reporting in the runners will treat the compares as if they're `Group`s with two elements.