Automatic split layers and tags when --save-as receives {layer} or {tag}

[dacap]

We can automatically interpret the given file name in --save-as and iterate over layers and tags to save each layer and tag as different files. For example:

aseprite -b input.ase --save-as output-{layer}.gif

Should to the same as:

aseprite -b --split-layers input.ase --filename-format {path}/{title}-{layer}.{extension} --save-as output.gif

[dacap]

Fixed, this will be included in v1.2-beta1