add support for named fonts

[Kiyoshika]

While exploring this crate for a project, I noticed there was no support for custom fonts (only system fonts.)

After reading through the AppKit docs and the source code a bit, it was pretty straightforward to add. I've also updated the safe_area example which just prints "Hello Word" in two additional custom fonts; wasn't sure a more relevant example to put this.

(NOTE: the trailing commas were added by my editor's formatter, if we really want them removed it won't be a big deal)