Adds an option to inset the legend.

To use — set the position to “inset”. Then, configure the inset
position by setting anchor to “top-left, top-right, bottom-left, or
bottom-right”, and offset x and/or y.

For example:
legend: {
     position: "inset",
     inset: {
     anchor: "top-left",
          x: 20,
          y: 20
     }
}

Merge branch 'feature/autoreduce' of https://github.com/danelkhen/c3 into danelkhen-feature/autoreduce