Add options to set legend placement

[moromis]

Currently the legend is fixed in where it displays. This feature intends to add a parameter to Clustergrammer called legendOptions which would allow the user to change things like whether it's horizontal or vertical, min-max or max-min, and where the legend is positioned. Other options can be planned as comments on this issue.

(originally suggested by @WenheLI here)

[moromis]

Added some options in these commits: #1 #2 #3, currently you can pass:

legend?: {
  show?: boolean;
  x?: string | number;
  y?: string | number;
  side?: "left" | "right";
  height: string | number;
};

I'd like to add allowing for top-bottom, which would automatically make it go sideways. After that I think this could be called closed.