Added a command line switch for turning on "Show transparency as white" by default

Thanks for your great program! I'm using it on arch linux with awesome wm, and it really is a godsend for tiling WMs.

This patch is useful to me, as I have a ton of pngs with transparency for publication, and manually switching transparency to white after every time opening an image can be a hassle. Now I (or anyone else) can just do alias sxiv="sxiv -A" and be done with it. Shouldn't change the default behaviour of the program. I hope I got all the help and manpage bits correct.

Matthias