add exceptions to leadingUnderscoreAccess

[qfox]

• __constructor
• __proto__
• etc.

see also https://github.com/jscs-dev/node-jscs/blob/master/lib/rules/disallow-dangling-underscores.js#L24

Add option to add your own exceptions.

[qfox]

Partially done. Need to add allExcept here.