Add feature to flag last comic as deleted

[SylvainDe]

It happens quite a lot that a change on a webcomics leads to issue that gets fixed by deleting the last comic in the json file (this happens when a post gets removed from instance). The workaround works because all the update process is based entirely on the last comic in the "DB".

It would be nice not to have to open a text editor to do so and do it directly from the CLI. Also, I'd like this not to actually remove the comic from the file but just to add a flag (much like what is done for new comics) so that I can try to undertstand the real issue later on.

This means that the comic abstract class needs to have a "public" method to get the list of (non-deleted) comics and one "private" method that gets all comics (including deleted ones) so that they can be saved back in the file after an update.