improve font handling

[derf]

right now feh has a default font and the user needs to specify --fontpath to load other ones. this sucks. So change the default font to DejaVuSans or something, and figure out the paths without user interaction (fontconfig perhaps?). judith font only as fallback.

(I'll do this myself, this is just so I don't forget it)

[stellarhoof]

Sprinkle a bit of Xresources on top if you can. But this def should be done.

[bheywar]

Any word on this? Looking for the ability to set a default font, as my resolution is weird and the current default font is way too small for me.