Add typings for TypeScript support

[hossam-magdy]

This PR adds/declares the detailed types based on the documentation, in index.d.ts so that the library would be TypeScript friendly.

Resolves issue #32

[Matthew-Smith]

The import should be outside of the namespace declaration:

import { Transform } from "stream";
declare namespace CsvString {

[touv]

Thanks,
see the new release : [email protected]