delete unwanted options from map transforms

[Rich-Harris]

This relates to babel/gobble-babel#2 - a map transform can specify default options like so:

function myTransform ( code, options ) {
  // ...
}

myTransform.defaults = {
  // we need these for some map transforms, but we don't want to
  // pass them through to the underlying library or otherwise use
  // it inside the transform
  accept: [ '.yaml', '.json' ],
  ext: '.js',

  // we probably *do* want this to be passed through
  somethingElse: 42
};

This changes the behaviour such that ext and accept are removed from the options object passed into the transform. (This might seem slightly arbitrary, but map transforms are already a layer of opinionation on top of 'standard' transforms, and the convenience outweighs the potential pain, IMO.)