Added override to use direct "PostScript" names for fonts

In order to fix #804 I added a ""-coretext-fontname" style element, it can be used to specify the postscript name of a font to use it. It was only a temporary fix but I thought it might be useful to other as well. 

I didn't build any check to see if the postscript name has an actual font installed, so this might crash if you use a font that is not in your bundle. Feel free to improve on this.