Add manual mode for palette overrides

[grunt-lucas]

This feature will be really useful for users who want to do fully manual specified palettes. When palette override manual mode is enabled, Porytiles will require that all palettes have a specified override, and that all slots in the override are filled, i.e. no - wilcards allowed. Porytiles can then give really useful error messages when there is no palette available for a specific tile. It could show something like:

error: metatile 12, layer middle, subtile northwest had no matching palette
note: 01.pal was a close match, but was missing:
    16, 80, 0
note: 04.pal was a close match, but was missing:
    132, 96, 32
    240, 232, 48

Enable via:

--pal-override-mode=manual