add entrypoint config option

[thepassle]

Currently we scoop up the entrypoints based on the package.json, but it can also be helpful to be able to run it on any arbitrary file. Something like:

npx barrel-begone --entrypoint=foo.js
# or
npx barrel-begone --entrypoints=foo.js,bar.js

Or

export default {
  entrypoints: ['foo.js']
}

Should be a pretty small change