Add a --filename-format option to specify generated file names in certain conditions

[dacap]

There are a a couple of command line options which generate file names (--data and --save-as when used along --split-layers). It would be great to have an option (e.g. --filename-format) to specify this mini-filename template.

At the moment, the fixed file name format is something like

{path}/{title} ({layer}) {frame}.{extension}

[dacap]

Available variables for the format:

• {fullname}: full file name with path and extension
• {path}: directory where the file is located (without trailing /)
• {name}: file name and extension (without path)
• {title}: file name only, without extension and without path
• {extension}: extension only (without .)
• {layer}: layer name
• {frame}: frame number (starting from 0)
• {frame1}: frame number (starting from 1)
• {frame00}: frame number (starting from 00)
• {frame001}: frame number (starting from 001)
• etc.