Add support for TypeScript

[matsprea]

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I think it would be nice to have TypeScript type definitions for Tangram.

This is a good guide to how to write definition for a js library.

Finally the definition should be published also on a popular repository such as http://definitelytyped.org/ for easy of access

[bcamper]

Hi @matsprea, I'm always been intrigued by TypeScript, but at this point we have chosen to invest in ES6 and beyond since it's standards tracked. I still think there are a lot of interesting things going on in the TypeScript community though. We'd be happy to see a type definition for Tangram and would welcome a community contribution for this, but it's unlikely we'll get to it on our own anytime soon.