Add 'goal clerk close' subcommand

## Summary
The `goal clerk send` command has multiple modes which lead to overly complicated CLI arguments. Specifically, you must always set `amount`, `to`, and `close-to` fields to close an account. In reality `amount` and `to` should be optional.

## Scope/Requirements
We should add a `goal clerk close` subcommand which allows for this simplified workflow. It would have the following flags:
* `close-to` - required, close the account to this address.
* `amount` - optional, defaults to zero.
* `to` - optional, defaults to `close-to`.

## Urgency/Relative Priority
This is not urgent.