Add prefix to extra fabulous comics to avoid conflicts
Now, the 'local_img' part matches exactly the content of 'img' with eventual NULLs

To make the retrieval of missing resources easier, NULL is now inserted in 'local_img'
if the dl failed. It also makes things easier to track to see what went wrong.

Took this chance to improve the validity checks we perform.
Make the CLI more usable with argparse - temporary interface

One can update which comics to update by providing arguments