Allow modifying props in custom render function

[bengry]

Continuing work done in #83 by @MarcOne68.

Note that this is only a partial replacement for Fabric's pattern:

onRenderText={(props, defaultRenderer) => {
  // no change
  return defaultRenderer(props);

  // props change (support added in this PR)
  return defaultRenderer({ ...props, text: props.text.toUpperCase() });

  // template change (not supported, since we want to abstract away JSX from consumers of `@angular-react`
  // this is supported by passing in a `ComponentRef` or a `TemplateRef`, but then you lose the ability to use `defaultRenderer`.
  return (
    <div style={{ color: 'red' }}>
	  {defaultRenderer(props)}
    </div>
  );
})}

since we still want to abstract away JSX from consumers, and Angular doesn't have first-class support for creating templates in TypeScript.

There are several ways to go about full solutions to this, but they are out of the scope of this PR