Implement getting a Budget by its ID

[phrohdoh]

Similar to Client::get_category_by_id we want to be able to get a Budget by id.

The URL to make a GET request to: /budgets/{budget_id}

This implementation will basically be a copy and paste of the get_category_by_id implementation.

See the api docs for more information.

Also implement the relevant CLI command using the existing commands as reference.