Allow list downloading when lists already exist

[marty-oehme]

Currently, once there are lists existing in the bemoji data directory, the program will never download additional lists.
This holds, even if you specifically supply the program with the -D option which tells the program which lists you would like downloaded.

That means, for most people, after running the program once they will not be able to download additional lists and the -D option becomes largely useless.

What would improve usability instead, I believe, would be to change -D into a command switch, akin to -t, -c and -e currently, which starts a download of all lists supplied - no matter the current state of the data directory.

This will allow downloading additional lists at any point and makes the -D switch a little less useless after the initial run.