Add support for the --quality flag to specify image quality when
resizing jpeg images.

This commit changes the behavior of the '-q' and '-v' command-line flags.
Instead of 'quiet' the '-q' flag now sets the image quality. To set the quiet
mode use the longer '--quiet' flag. The '-v' flag has been dropped
completely, instead use '--version'.

Thanks to @ArtisAvotins for the PR. I took his initial work and cleaned
things up a bit. It also encouraged me to fix some other issues with the
script.

Closes #7.