Core.py: Move join_names method from ConsoleInteraction.py
Move join_names method from coala/coalib/output/ConsoleInteraction.py here.
Related to https://github.com/coala/coala/issues/3486
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Move join_names method from coala/coalib/output/ConsoleInteraction.py here.
Related to https://github.com/coala/coala/issues/3486