Add TypeScript type definitions

[JDDurrant]

I would like to add TypeScript type definitions to this library.

The documentation is pretty good, but I'm not a fan of alt-tabbing between my project and the library docs.

I might also want to move some of the documentation into JSDoc/TSDoc comments, so my editor will pick them up.

[JDDurrant]

@johnwalley Would you be against the source code for this library being converted to TypeScript?