Should not overwrite existing kata_files

[janernsting]

codersdojo overwrites existing kata_files when executing the setup. This happens, because kata_files are copied from its template folder.

To better support legacy code katas, it would be great, if codersdojo setup would check to see if a kata_file already exists and provide an according message as to inform the user about this.

One option could be to add one flag, which ensures that existing files are not overwritten. Otherwise codersdojo should always overwrite existing files (currently the default).