`codeunion config` should support appending to list-type values

[jfarmer]

I imagine it working something like this:

$ codeunion config user.repositories foo bar
$ codeunion config user.repositories
foo
bar
$ codeunion config user.repositories --append baz
$ codeunion config user.repositories
foo
bar
baz

[jfarmer]

Removing from first student release because we have yet to find an instance where this needs to be supported by any of the accompanying programs.