Export and Import metrics

**Issue by [BrianHicks](https://github.com/BrianHicks)**
_Friday Feb 10, 2017 at 16:18 GMT_
_Originally opened as https://github.com/BrianHicks/elm-benchmark/issues/5_

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we should be able to export benchmarks for safekeeping and comparison against new data. This should be independent from running benchmarks (and possibly related to #4.) My ideal workflow:

1. Run benchmarks
2. Save data (CSV, JSON, whatever… as long as it's consistent)
3. Improve code
4. Import prior data for analysis

Also:

1. Run benchmarks and save data ("a")
2. Run improved benchmarks and saved data ("b")
3. …
4. ~profit~ import benchmark data sets a and b for analysis.

This may mean that the analysis may be separate from the benchmarks. That wouldn't be the end of the world, and could pretty easily be hosted somewhere static and public.