Add type definitions

[demurgos]

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 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 gulpjs/async-done#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.

[phated]

@demurgos I'm open to typings but not sure how I feel about rewriting the code in typescript. If we were going to rewrite in a language, it'd probably be ReasonML (which can also generate typescript definitions).

[demurgos]

Ok, I'll just send typings then.