Adds the option `-t, --type <filetype>` which can be used to select the
filetype of the produced image/video.
This will only take effect if you don't pass a filename to swaygrab e.g.
let swaygrab use the default date based filename.
With this it's possible to run `swaygrab` without a filename argument.
With no filename supplied it will use a default name based on the
current time.
The default file will get the extension `png` for screenshots and `webm`
for video capture.
Using 'flag' results in duplicate code paths for short and long options.
This broke the -q short option in swaymsg, because there was:
{"quiet", no_argument, &quiet, 'q'}
Which will set quiet to 'q' and return 0, not 'q'.