Add support for +W### option in comments to unignore an warning

Allows a warning to be ignored for a number of lines, and then
unignored after. For example:

    var y = Object.create(null);
    // ...

    //jshint -W089
    for (var x in y) {
        // ...
    }
    //jshint +W089

suppresses the warning about not wrapping a "for in" in an if in this
case, but leaves the warning enabled for later occurrences in the
file.