Add type definitions

I'd like to use this library as a dependency of a Typescript project but there are no type definitions currently (even on DT) so it's harder to integrate. It would be nice add types here. (See [my plugin-error issue](https://github.com/gulpjs/plugin-error/issues/5) for other reasons it may be a good idea).

Having the types here would also allow the Gulp type definitions to pull its `watch` options from here instead of redefining them downstream.

I'll send a PR myself to fix this issue (probably after https://github.com/gulpjs/async-done/pull/47 is merged). Given that the library is very short it may also be possible to just convert it to TS and generate the types.