Let GTK handle command line arguments.

Hi,

I let GTK handle the command line arguments. It will give a little more nicely formatted help text (at least in my opinion). Also, the use of getopt can be dropped in favour of GTK and less code.

With this additional long option support is added (e.g --fullscreen).

I also updated the README.md to use a GTK'ish formatting, i.e, the formatting shown when --help flag is passed.

Please see what you think and merge thereafter!