Add load_os_font_list for linux in text-freetype2 plugin

This implements searching for available fonts on linux using
the fontconfig library.
Fix crash in text-freetype2 plugin

There seem to be some ancient font files out in the wild that don't
specify a font family, which may cause a crash when family_in is NULL.
This adds a NULL pointer check to add_font_path to avoid crashing.
Merge pull request #1 from fryshorts/fixes

Fixes