Support `__label__` region in .p8 files
Pico-8 0.10 added a new `__label__` region. picotool rejects this when it encounters it. picotool should support `__label__` fully, including preserving and using the label when converting between .p8 and .p8.png. As a partial measure, it should ignore the `__label__` region instead of rejecting it.