Add possibility to use system font

[VitaliyR]

Hello!

Thanks for your library, it is really helpful. While I've played with it, I've encountered that I can't use system font in tags. Actually, I probably can via:

<font \(UIFont.systemFont(ofSize: 24).asAKAttribute())>

But it is too long and not readable for me.

I'm proposing to make fontName optional, so if it not provided - use system font.
What do you think about this?