Add ignore feature so that some special require statement can be ignored.

The use case is when I am writing my own date-util.js which requires moment.js,
one speical require statement in moment.js cannot be handled by webmake:
require('./lang/' + key);

The purpose of wrapping moment.js to my own libary is for easier swapping to
another libary in the future if needed.

Hence, ignoring that special require and manually include some language file
can be enough.