Improve parsing of default and initial value

Summary

Vuedoc Parser can be improved by adding parsing of UnaryExpression and improving output of default value.

Improved Interface

interface PropEntry extends Entry {
  kind: 'prop';
  name: string;               // v-model when the @model keyword is attached
  type: string | string[];    // ex. Array, Object, String, [String, Number]
  nativeType: NativeTypeEnum;
- default: any;               // '__undefined__' value for uncatchable value
+ default?: string;
  required: boolean = false;
  describeModel: boolean;     // true when the @model keyword is attached
}

interface DataEntry extends Entry {
  kind: 'data';
  name: string;
  type: NativeTypeEnum;
- initial: any;               // '__undefined__' value for uncatchable value
+ initial?: string;
}

type MethodParam = {
  type: string;
  name: string;
  description: string;
- defaultValue: any;
+ defaultValue?: string;
}