Ability to add custom fields to offer form. #199

 add_action('make_offer_form_before_submit_button', 'add_custom_field_make_offer_form', 10);
    add_action('make_offer_after_save_form_data', 'save_custom_field_make_offer_form', 10, 2);
    add_action('make_offer_after_buyer_meta_display', 'display_custom_field_buyer_section');
    add_action('make_offer_email_display_custom_field_after_buyer_contact_details', 'display_custom_field_after_buyer_contact_details', 10, 1);

    function add_custom_field_make_offer_form() {
        ?>
        <div class="woocommerce-make-offer-form-section">
            <label for="offer_postcode"><?php echo __('Postcode / Zip', 'offers-for-woocommerce'); ?></label>
            <br><input type="text" value="" required="required" name="offer_postcode">
        </div>
        <?php
    }

    function save_custom_field_make_offer_form($post_id, $post) {
        if (isset($post['offer_postcode']) && !empty($post['offer_postcode'])) {
            add_post_meta($post_id, 'offer_postcode', $post['offer_postcode']);
        }
    }

    function display_custom_field_buyer_section($post_id) {
        $offer_postcode = get_post_meta($post_id, 'offer_postcode', true);
        ?>
    <li><span><?php echo __('Postcode / Zip:', 'offers-for-woocommerce'); ?>&nbsp;</span><?php echo (isset($offer_postcode)) ? stripslashes($offer_postcode) : __('Missing Meta Value', 'offers-for-woocommerce'); ?></li>
    <?php
}

function display_custom_field_after_buyer_contact_details($post_id) {
    $offer_postcode = get_post_meta($post_id, 'offer_postcode', true);
    echo (isset($offer_postcode) && !empty($offer_postcode)) ? '<br /><strong>' . __('Postcode / Zip:', 'offers-for-woocommerce') . '&nbsp;</strong>' . stripslashes($offer_postcode) : '';
}