add CLI command to list existing topics

[yurytsoy]

Candidate tools:

• https://pypi.org/project/click/
• https://github.com/Textualize/rich
• https://github.com/google/python-fire
• https://github.com/prompt-toolkit/python-prompt-toolkit

[yurytsoy]

Examples of usage:

• run using default config (hmeg.conf): python hmeg.py
• run using custom config: python hmeg.py -c configfile
• run using custom topic and number of exercises: python hmeg.py -t "topic" -n 10
• list topics from default config: python hmeg.py -l
• list topics from folder: python hmeg.py -l -f "hmeg/topics/"