Support the item ID field (like "1.3.5")

[erjosito]

We are going to add a new field to the checklist items that will identify checklist items (see Azure/review-checklists#459). For example, if a given ID is 1.3.5, 1 would identify the category, 3 the subcategory ID inside of that category, and 5 the item ID inside of that subcategory.