Add JSON output mode

[darshan-Jahagirdar]

Fixes #1.

Changes

• Added --json to add, list, and done commands.
• add --json prints the created task object.
• list --json prints an array of tasks, or [] when empty.
• done --json prints the updated task object, or an error object when no tasks exist.
• Added README examples for JSON output.

Verification

• python -m pytest test_task.py -> 4 passed
• CLI smoke test with a temporary home directory:
  • python task.py add "Buy groceries" --json
  • python task.py list --json
  • python task.py done 1 --json

/claim #1

[darshan-Jahagirdar]

Validation update for this PR:

ext py -3.11 -B -m pytest test_task.py git diff --check

Result: 4 tests passed and the diff whitespace check passed. I installed pytest into a temporary test-only directory for this run and removed it afterward.