Getting task metrics from CLI/API

[inahga]

Right now the task metrics are present by listing all tasks, i.e. divviup task list which invokes GET accounts/{account_id}/tasks, but this doesn't refresh metrics (and it shouldn't).

To refresh metrics, we need to invoke GET tasks/{task-id}. There is no CLI functionality wired up for this. We could allow divviup task get {task-id} or thereabouts.

(I think there's a better design for acquiring metrics, but I'd like to keep necessary changes minimal as we evaluate how useful the metrics are).