Simplify TagCommand by eliminating the subsubcommands.
Throw InvalidTodoException when the regex doesn't have exactly 1 hit.
Add identfication with regexps in TagCommand.
Add test for an invalid regexp.
Eliminate convert_todo_number.

Commands shouldn't care about the format of the todo number, TodoList
will take care of that now.

Added a bunch of tests for the commands to make sure they can deal with
regexp values.