Move assi_setFontDir into assi_init.

ass_set_fonts_dir must be called before ass_set_fonts, otherwise it
won't do anything interesting. Since ass_set_fonts must be called in
assi_init or it's possible fonts will simply be uninitialized,
ass_set_fonts_dir must be called there, too.
Add example fonts.conf

Shamelessly stolen from lachs0r's windows mpv builds.