[Bug Report] v1.5 release does not read default.ini correctly.

[tai271828]

Steps to Reproduce

1. install the package from PyPI by pip install pycontw-report-generator
2. Prepare the test data (ticket csv etc.)
3. Execute the following command

rg-cli --talks-csv ${HOME}/work-my-projects/pycontw-projects/post-events/2020/data/misc/talks-list.csv --proposed-talks-csv ${HOME}/work-my-projects/pycontw-projects/post-events/2020/data/misc/talks-list-proposed.csv --booth-csv ${HOME}/work-my-projects/pycontw-projects/post-events/2020/data/misc/booth.csv --csv ${HOME}/work-my-projects/pycontw-projects/post-events/2020/data/tickets/with-identities/individual-attendees.csv --csv ${HOME}/work-my-projects/pycontw-projects/post-events/2020/data/tickets/with-identities/reserved-attendees.csv --csv ${HOME}/work-my-projects/pycontw-projects/post-events/2020/data/tickets/with-identities/corporate-attendees.csv --yaml ${HOME}/work-my-projects/pycontw-projects/post-events/2020/data/yamls/general-plots-en.yaml --package-yaml ${HOME}/work-my-projects/pycontw-projects/post-events/2020/data/yamls/packages.yaml --sponsor-yaml ${HOME}/work-my-projects/pycontw-projects/post-events/2020/data/yamls/sponsors.yaml
Traceback (most recent call last):
  File "/home/tai271828/.local/share/virtualenvs/demo-post-event-01-gQWD0w3T/bin/rg-cli", line 5, in <module>
    from report_generator.controller.report_generator_cli import main
  File "/home/tai271828/.local/share/virtualenvs/demo-post-event-01-gQWD0w3T/lib/python3.6/site-packages/report_generator/controller/report_generator_cli.py", line 79, in <module>
    show_default=True,
  File "/home/tai271828/.local/share/virtualenvs/demo-post-event-01-gQWD0w3T/lib/python3.6/site-packages/click/decorators.py", line 247, in decorator
    _param_memo(f, OptionClass(param_decls, **option_attrs))
  File "/home/tai271828/.local/share/virtualenvs/demo-post-event-01-gQWD0w3T/lib/python3.6/site-packages/click/core.py", line 2465, in __init__
    super().__init__(param_decls, type=type, multiple=multiple, **attrs)
  File "/home/tai271828/.local/share/virtualenvs/demo-post-event-01-gQWD0w3T/lib/python3.6/site-packages/click/core.py", line 2101, in __init__
    ) from None
ValueError: 'default' must be a list when 'multiple' is true.