Get the value of the attribute at `path` based on the `options.defaultValue`, and `options.valueRegExp` if present.
function(path, options) {

        var cell = this.options.cellView.model;

        var value = joint.util.getByPath(cell.attributes, path, '/');
        if (_.isUndefined(value) && !_.isUndefined(options.defaultValue)) {
            value = options.defaultValue;
        }

        if (options.valueRegExp) {

            if (_.isUndefined(value)) {
                
                throw new Error('Inspector: defaultValue must be present when valueRegExp is used.');
            }
            
            var valueMatch = value.match(new RegExp(options.valueRegExp));
            value = valueMatch && valueMatch[2];
        }

        return value;
    }