.p8 writer should minimize data sections

[dansanderson]

A while back Pico-8 extended the .p8 format to allow for omitting trailing lines from the data regions that only contribute default (empty) data. picotool should do the same.

By surprising coincidence, picotool doesn't have an issue reading files in the extended format, and Pico-8 doesn't mind trailing default data. So there isn't actually a compatibility issue here. It'd just be nice if picotool did the same thing that Pico-8 does when saving files.

[dansanderson]

Dupe of #61.