getTiddler = tiddler =>
  $tw.wiki.getTiddler(tiddler instanceof $tw.Tiddler ? tiddler.fields.title : tiddler)
Similar to {@code wiki.getTiddler()} but also accepts a tObj as
argument, thus, making it unnecessary to always differentiate or remember
if we are dealing with an object or a reference.

@see https://github.com/Jermolene/TiddlyWiki5/blob/master/boot/boot.js#L866
@param {Tiddler} tiddler - A tiddler reference or object.
@return {Tiddler} A tiddler object.