Allow non-centered wallpaper images (align left/right/offset)

Does what it says on the tin, with man page update included. Overloads the --geometry option for wallpaper setting (no added options!) to allow for specifying offsets/alignments for wallpaper setting.  For instance, if one wanted, say, [this photo](http://legault.perso.sfr.fr/atlantis_2009may12_crop.jpg) maximized but aligned left, one could now do that with:

    feh --bg-max --geometry +0+0 ~/Wallpapers/shuttletransit.jpg

Of course, in --bg-max the vertical offset will have no effect if the constraining dimension is height.  Under --bg-center, both parameters take effect.  It currently only supports those two, it could perhaps be interpreted for the other options as well, but it made the most sense to me for the two that rendered black borders.

Let me know of any feedback/suggestions - I tried to match existing code conventions/style, but there are several ways it could be implemented (extra variables vs repetitive code, for instance) so I'd be glad to rearrange things.  If y'all think it makes sense to add it to the other options as well, I'll look into it - hell, I may do so anyway and add it here.