Simple implementation of iOS Dynamic Type with custom fonts.
The purpose of this class is to support Dynamic Type for a custom font. The goal is to make the implementation as simple as possible. Just set a label's class to this subclass in the Interface Builder inspector. You don't need to also set the IBOutlet type to this subclass, it can remain a UILabel. You can also use this subclass in code instead of UILabel. This is implemented as a subclass rather than an extension because we need to keep a reference to a label's original font to support cell reuse in a scrolling table view.