Support typescript 3 style DefaultProps

[hipstersmoothie]

In TypeScript 3.0 the way you can specify default props on a component is as follows:

import React from 'react';

interface MyProps {
  foo: string;
}

const MyComponent = (props: MyProps) => {
  // implementation
};

MyComponent.defaultProps = {
  foo: 'bar'
};

export default MyComponent;

Now inside MyComponent the prop foo will be of type string but to the user of the component it will be type string | undefined.

Currently in react-docgen-typescript the above will produce a props table that states foo is required, even though this is not the case.

My changes mark a prop as required only if it not optional and there is a code based default (i.e not a jsdoc default since that code wouldn't type check).

closes #129

[pvasek]

Thanks! Published on npm as v1.14.1