Add TypeScript type definitions

[Grzesie2k]

It would be nice to add TypeScript type definitions.

[arnthor3]

Could you do a pull request? That would be awesome, I am not a typescript guy so this would require some research..

[axesoota]

@arnthor3 : Could you please publish the changes to npm now that the PR has been merged?

[Lilleri]

complete the PR to add the typescript support?