[dwm] start maximized pull request

This is my very first pull request.
And my very first post on the list! :)
The pull request allows ppl to have chosen window class start maximized.

I have a tag for alleast every program I use, so I can always go full
screen. Some program does not save his size on exit (ex. xterm),
so every time it starts I have to maximize the window.
Thats frustrating when you start lots of programs with a startup script.

With this pull request, Rule struct gets a "ismaxed" bool which can be set to
true/false through config.h. Quite simplistic.

Wyrmskull