Allow adding deb lines through add-apt-repository

Checking for faulty lines is not implemented yet, and it also doesn't yet add the deb-src line if source repos are enabled. I likely don't have time to implement those because of my vacation.
If I have not added these features by Friday, 26 July, you can merge the pull anyway: in its current state the code at least works for those people who get add-apt-repository commands from online tutorials.

The repository is added in the additional-repositories.list file, unlike ubuntu's version of add-apt-repository which added it in sources.list itself.

Line 63 had a trailing whitespace which is not shown by github.

I have renamed the add_ppa_via_cli function because it doesn't only add PPAs but also other sources now.