Feature request: add loop mode for --snapshot

[DirtYiCE]

Normal top has -b (and -d) to have it print info to stdout periodically, nvidia-smi has --loop to do the same, could nvtop's snapshot gain the same functionality? (And maybe have it output a single line for easier interop with line based tools). It would save me from spawning a new process and doing all the initialization logic periodically (which is especially painful on nvidia cards, where a single nvtop -s run can take 1.5 seconds with ~80% SYS CPU usage during that period).

[busybox11]

Also, this would fix Intel GPU stats not being reported. They work fine in the regular view but init with N/A, which is reflected in snapshot mode by null.

Might be a subject for a different issue though, that being said it would help for this usage right now without having to do anything special for Intel.

EDIT: Seems like this is related to #371, sorry for double-posting!