Add new `addtext` command

Hey! First of all - nice one! Etherdump is rad. :)

We're going to use this for pad.vvvvvvaria.org and we have the situation that we would like to first exclude all pads from the archive (using the __NOPUBLISH__ marker) and then selectively remove the marker and archive them as we go. It would be nice to have a command that automatically appends a custom message and the marker to all pads.

It could look like: (Instead, see #1 (comment 895))

$ etherdump nopublishall [--nopublish] "My custom message"`?

I thought it might be nice to upstream that here since it might also be useful for others who may have many existing pads for private uses and later want to use etherdump. Existing python API clients seem out of date and a command to do this on the database level is not always accessible to others (my other approaches ... ).

What do you think? If we're in agreement, I'll get working on it.