Negative numbers as default values are not parsed

[flash1293]

If a negative number is defined as the default prop value, it is ignored in the styleguidist output, e.g.

interface TestProps {
  numProp: number;
}
export Test extends React.Component<TestProps, {}> {
  static defaultProps: TestProps = {
    numProp: -1
  };
 //...

This is due to the parsing in parser.ts#getLiteralValueFromPropertyAssignment - the switch-case-block handles SyntaxKind.NumericLiteral, but the - from -1 is passed as SyntaxKind.PrefixUnaryExpression

[pvasek]

Thanks to @dotcs fixed in version v1.2.3.