Allow for optional merging of Object type option props

Augment `this.options()` method with one more argument to allow for properties of type Object to merge instead of overwrite.

case:

``` js
multiTask: {
options: {
anObject: {
a: 1
}
},
target: {
options: {
anObject: {
b: 2
}
}
}
}

// in the task
grunt.registerMultiTask('multiTask', function() {
var options = this.options({}, true);

// desired result
// options.anObject === {a:1, b:2}

});
```