[feat] add the comma output format

[4383]

These changes add the comma output format where results are represented on one single line and separated by commas.

Also these changes:

• fix an issue with newline output when elements are not instance of string type.
• reindent properly the help format's part
• rename the unit test module related to the ouput module

How to test:

Setup your env:

$ git add remote hberaud git@github.com:4383/niet
$ git fetch hberaud
$ git checkout -b comma hberaud/comma

Now run tests:

$ # Example of display
$ tox -e venv -- niet "oslo.deliverables.*.repos[0]"
    https://raw.githubusercontent.com/openstack/governance/master/reference/projects.yaml
    -f comma
$ # Example of integration with beagle and the openstack governance data
$ python3 -m venv .venv
$ source bin/activate
$ pip install beagle
$ beagle search -f link --repo $(niet
   "oslo.deliverables.*.repos[0]"
   https://raw.githubusercontent.com/openstack/governance/master/reference/projects.yaml
   -f comma) 'venv'