Enable values array to contain objects with metadata

[nlively]

This PR allows values to be passed as an array of objects. As long as an object has a value property, the value property will be used to position the handle, and the rest of the object will be available as metadata. Metadata is available when rendering handles. Useful for specifying CSS classes, rendering custom tooltips and color coding handles

[sghall]

Hey. Thanks for making this PR. I like the idea. Just scanning it, I'm thinking the Typescript support might be an issue with the any.

This part:

values: ReadonlyArray<any>

Do you use Typescript? Thinking we need to provide a way to pass a generic so it's type safe. I know there's quite a few TS users of this library. Anyway, I'll take a look at this more closely when I have a moment.

[nlively]

@sghall, I'm probably not the best guy to incorporate typescript since that's outside my core competency. Perhaps someone else wants to pick up where I left off?