Feature: allow customizing buttons

[Fabilin]

Bots can currently display 3 types of buttons: URL buttons, post-back buttons, and quick replies, but the default component for these may not always be enough. For example, an application may wish to add a custom listener to one or more button types, to run custom effects or provide analytics, or it may wish to customize the button rendering with extra elements (like icons for specific links).

It would therefore be helpful to extend the renderers setting introduced in #159 for images and text with a new ButtonRenderer type.