Plain bundle format

[Rich-Harris]

This has suddenly become a priority (long story...). I think the best way to handle external imports etc is to simply disallow them - in other words the bundle has to be completely self-contained for this to work.

Something like

esperanto.bundle( options ).then( function ( bundle ) {
  var plainBundle = bundle.concat();
  fs.writeFile( 'bundle.js', plainBundle.code );
});