[dwm] [patch] mwfact per tag

Hello,

Someone on the IRC channel asked for a patch to remember the mwfact
per tag. This is a quick and dirty patch. NTAGS must be defined in
the config.h, and if multiple tags are selected it uses the one that
comes first in tags[]. I wanted to use ntags to define the size of
mwfact[], but can't due to compiler errors. I'm also not proud of the
way i initialize mwfact[]. Please feel free to solve these issues,
just let me know how you do, so I can learn and write better code next
time around.

-Evan