A to-do list manager

A command interpreter for your to-do list. Read commands like `add`, `done` and `remove` from input, keep tasks in a Vec of structs, give them stable ids, and parse every line into an enum before acting on it.

Milestones

1. Add and list
2. Check things off
3. Remove tasks, keep their ids
4. Parse commands into an enum
5. Polish: filters and a progress line