feature: automatically generate image from RunHistory

[hazcod]

Hi, I think it would be nice if we could generate an image from the JSON file to e.g. automatically display & replace the image on README.md on merge.

[bobheadxi]

Different export options from gobenchdata would be nice, but it might be a bit outside the scope of the project. Some options might be:

• generating an image based on the web UI (ie the example screenshot in #30)
• parsing benchmarks.json and generating your own visualization:

    b, err := ioutil.ReadFile(f)
    if err != nil {
        panic(err)
    }
    var runs []bench.Run
    if err := json.Unmarshal(b, &runs); err != nil {
        panic(err)
    }

I'm open to thoughts though!