Define different users per sudoer.

templates/sudoers.d.j2:
- Changed template so that it uses sudoers as a array of
dictonaries, each dict containing the sudoer name and the list of
users it can execute commands as. This can be futher expanded for
groups and commands. I felt the need to perform this change when
I tried to add sudoers that can access different users within the
same provision.

ansible-sudoers.yml:
- Added tests for this new functionality.

README.md:
- Changed README accordingly.