Adds graph UI on the homepage and allows users to select runner/benchmark to graph average scores

This shows two drop-downs on the homepage where users can select a runner and a benchmark they are interested in. A graph is then shown which contains the average of the selected benchmark by ruby version.

I haven't spent much time on the UI/UX as I just wanted to get the initial functionality in. Also I'm not wedded to the charting library, we can easily change that later.

I haven't added a features spec for this as I wasn't sure how best to do that. Any ideas would be great.

I think this is ok as a starting point but of course can be changed. As usual any comments are totally welcomed :smile: