Convert to TypeScript

[balupton]

Definitions already provided by @atd-schubert at:
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/5344bfc80508c53a23dae37b860fb0c905ff7b24/types/progressbar/index.d.ts

[balupton]

@atd-schubert would you be interested in undertaking this task?

[atd-schubert]

This seems to be an easy task, I will make a PR...