Feature Request: support the whole node.js API

I was trying to use this library as a filesystem for webpack, but the lack of support for functions like `mkdirp` makes me unable to do so. There's no point to me to use `unionfs` with `memfs`+`fs` if it doesn't allow me to at least use all of `memfs` functions: https://github.com/streamich/memfs/blob/master/docs/api-status.md

