Add small interactive program and fix save bug

The program can only add tasks, and only one at a time. It's still
useful to read the list, though.

Not sure if I want to build on it. I prefer to keep this module a
library.
Add `FileTaskList` class

I prefer this way of dealing with task files. Stateless methods to load
and save from arbitrary paths require the caller to remember the file
path and duplicate it everywhere.