Add modifierWhitelist option

[cgewecke]

Option accepts a list of strings (simple modifier names like nonReentrant) to exclude from modifier branch coverage.

This is useful for modifiers that perform non-branch preparatory work.

Ex: synthetix/Proxyable:optionalProxy

    modifier optionalProxy {
        _optionalProxy();
        _;
    }


    function _optionalProxy() private {
        if (Proxy(msg.sender) != proxy && Proxy(msg.sender) != integrationProxy && messageSender != msg.sender) {
            messageSender = msg.sender;
        }
    }