Include type definition for TypeScript usage

[jimsleon]

TypeScript becomes popular these days, so it will be great if type definition is included in the npm page so that it could be used in TypeScript projects.

[davidchambers]

I'll happily accept a pull request for this, @jimsleon. :)

[saikirangit]

We just created a file string-format.d.ts under src

declare module 'string-format' { const format: any; export = format; }

and used it as

import * as format from 'string-format';