Custom packageBase configuration

[guybedford]

Asset filtering in ncc will currently filter to only emit assets within the package base, with some other rules as well.

It would be useful to be able to customize this option, and this would also solve the bug in the latest now-node build.

Something like:

ncc('/path/to/pkg/file.js', {
  packageBase: '/path/to/pkg'
})

will then never emit files outside of the /path/topkg base.

[guybedford]

Implemented in vercel/webpack-asset-relocator-loader#56.