Expose postscript-style font names

[corranwebster]

A number of backends (Quartz, PDF, PS) expect fonts to be specified by postscript-style names (rather than providing a reference to a TrueType/OpenType file, for example). These names are available as part of the metadata in TrueType/OpenType files, so it would seem fairly straightforward to add a method to provide the font name in this format once the file is known.