Paramters transform: add loose option for default params

[hzoo]

Ref necolas/react-native-web#490 (comment) cc @necolas

Input

function bar1 (arg1 = 1) {}
bar1.length // 0

Current

function bar2() {
  var arg1 = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 1;
}
bar2.length // 0

Expected (loose option)

function bar3(arg1) {
  if (arg1 === undefined) {
    arg1 = 1;
  }
  // cannot change Function.protoype.length
}
bar3.length // 1

Change

• Adding a plugin option: https://github.com/thejameskyle/babel-handbook/blob/master/translations/en/plugin-handbook.md#plugin-options
• https://github.com/babel/babel/blob/7.0/packages/babel-plugin-transform-es2015-parameters/src/default.js
• update the readme
• tests with the loose option

[peey]

@hzoo I'm planning to add a "ignoreArity" option to #5751 . After it gets merged it's planned to be used by the parameters transform, so this functionality could be easily added there

[maurobringolf]

I started working on this, but its more of a sketch up to now. Feedback appreciated!