Use built-in Transform instead of through2

[talentlessguy]

through2 is outdated and there's no need to use it anymore. Streams are stable, and since this library is Node.js only, Node.js Transform could be used directly:

Since Node.js introduced Simplified Stream Construction, many uses of through2 have become redundant. Consider whether you really need to use through2 or just want to use the 'readable-stream' package, or the core 'stream' package (which is derived from 'readable-stream'):

(from through2 readme)