fix: copy over hooks from base schema

[vkarpov15]

Hi,

Thanks for the elegant lib! You're missing support for a couple things though, like copying hooks from the base schema (Automattic/mongoose#5897) .

An alternative implementation would be to do:

function extendSchema(schema, definition) {
  var clone = schema.clone();
  schema.add(definition);
  return schema;
}

This will copy over hooks and everything else, like plugins, query methods, statics, etc. Any thoughts on that?