wish: add-apt-repository needs to know distro code

This is a bug report for me to hang a patch off of.

I mainly want to avoid this $(bashism):
apt-add-repository 'deb http://packages.medibuntu.org/ '$(lsb_release -cs)' free non-free'

and softwareproperties/SoftwareProperties.py has .distro.codename, so I use it.

If it isn't clear, the new docs should:

+<sourceline> - The apt repository source line to add. This is one of:
+ a complete apt line in quotes,
+ a repo url and areas in quotes (areas defaults to 'main')
+ a PPA shortcut.

   Examples:
     apt-add-repository 'deb http://myserver/path/to/repo stable myrepo'
+ apt-add-repository 'http://myserver/path/to/repo myrepo'
+ apt-add-repository "https://packages.medibuntu.org free non-free"
+ apt-add-repository http://extras.ubuntu.com/ubuntu
     apt-add-repository ppa:user/repository