[e621] add keyword 'num' for post order in pool

[0x1F98E]

Pools in e621 have an explicit post order.

This PR exposes each post's index with the num keyword.

Useful for keeping comic pages in the correct order.

Example usage:

"pool": {
	"directory": [ "{category}", "pool", "{tags[artist]:J, /}", "{pool[id]} {pool[name]}" ],
	"filename": "{num}.{extension}"
}