[admin] Commands to append/remove values to config options

If a config option contains a list, we can use the .addto and
.removefrom commands to modify the list without having to retype the
whole thing.
[admin] Convert any option to a list when using .addto or .removefrom

Now uses config.get_list(), which currently converts the option to a
list on read.

As a result, no longer fails if used on an option with a non-list value.