Base2Tone-bat duotone colorscheme repo as submodule

[atelierbram]

I would like to see duotone color themes included in bat. These are only dark versions of Base2Tone colorschemes. Because these colorschemes have such a limited color palette, but come in lots of variations, it will make the choice for people to find something that they like and/or matches the look of their terminal emulator much easier imo.