WIP Add a electron-packager afterCopy hook to the exports

[MarshallOfSound]

This adds a createPackagerCopyHook function to the exports of electron-rebuild.

Basic usage is like so

packager({
  ...
  afterCopy: [createPackagerCopyHook()]
  ...
})

You can add custom options like so

createPackagerCopyHook(distUrl, ignoreDev, ignoreOpt, preGypFix, verbose)

This makes rebuilding modules for distribution so much easier as you don't have to worry about your node_modules folder being built for a different arch after you run your packager.

Fixes #99

[anaisbetts]

@MarshallOfSound Do we still want to do this?

[MarshallOfSound]

Nope, the newly exported function is already basically in the required form for an afterCopy hook. This has been superceded 👍