Add options to the script for different modes

[talkasab]

Make the script do everything by default, but make it so you can choose single actions with command-line flags (using click library):

• Only check all definitions are valid and no collisions (no name collisions, no ID collisions, no synonym collisions)
• Only re-generate index.jsonl, people.jsonl, and organizations.jsonl
• Only re-generate index.md and all the individual text/*.md files