Support plugins

[inorganik]

Regarding discussion #306, this PR makes CountUp support plugins. A plugin is a class instance or object passed in options that implements a render method:

export declare interface CountUpPlugin {
  render(elem: HTMLElement, formatted: string): void;
}

The plugin's render method will be called on each frame to display the formatted value instead of CountUp's.

[inorganik]

FYI @msoler75