Ability to add multiple layers in one call in beet.add

[zya]

At the moment if you have multiple layers, you will need to call beet.add multiple times. It would be nice if I could do something like this:

var layer1 = beet.layer(pattern1, cb);
var layer2 = beet.layer(pattern2, cb2);

beet.add(layer1, layer2);
//or
beet.add([layer1, layer2]);

[zya]

Fixed in v1.7.0