feat: config option 'overrides' to override palette colors

[gaoDean]

First of all, I love this variation of gruvbox, works better than any other out there, thanks for putting in the effort to create this.

I didn't like the dark blue in the palette, so this allows you to override any palette color with another color

vim.g.gruvbox_baby_overrides = {blue_gray = "#aaaaaa"}

or using existing palette colors

vim.g.gruvbox_baby_overrides = {blue_gray = "light_blue"}

[luisiacc]

Excellent suggestion, thank you!

Just a few details and we can merge this.

[gaoDean]

all good?