Support TypeScript Prop types

[sem4phor]

The problem

Since this PR is merged Vue supports prop declaration with typescript types (Without using the class-based syntax). for example:

interface SelectOption {
  label: string;
  value: string | number;
}

export default Vue.extend({
  props: {
    value: {
      type: [String, Number] as Prop<SelectOption['value']>,
      default: undefined
    }
...

The vue-docgen-parser returns undefined in this case resulting in types being not documented in the generated styleguide.

Proposed solution

Extend the parser to support the Typescript prop types when using the Vue.extend syntax.