jhatwich/layer comps+size

To use this feature add a layer comp and name it something like "overview.png" or "200% overview.png"
@iwehrman I don't know if this is the case, but it should be said that `layers.length - 1` only works when document is flat, without any layerSections, that can "hide" indexes inside. For this I think that we can rely on Photoshop that it will start indexes on zero and last index will always be `document.layers[0].index` as it is the top-most layer and layers are by design sorted by indicies