A simple shortcode that displays the “Total” filed value from any Gravity Form.
Parameters
* form_id
* field_id
* start_date
* end_date
Examples:
* [gravityform-total-amount]
* [gravityform-total-amount form_id="14"]
* [gravityform-total-amount form_id="14" field_id="18"]
* [gravityform-total-amount form_id="14" field_id="18" stat_date="2017-07-18"]
* [gravityform-total-amount form_id="14" field_id="18" stat_date="2017-07-18" end_date="2017-07-20"]