Typescript definitions

[thany]

I took the liberty to contribute definitions to DefinitelyTyped and, proud as I am to have done so, perhaps it is a good idea to integrate definitions in this package in a future update. If you do, feel free to take my contributed definitions as a starting point or blatantly copypaste them. You have my permission 😀

You can also decide to leave everything as-is, and require typescript users to also install @types/jconv as a devDependency for the time being. Up to you.