Add support for setting arbitrary colors ranging from 16 to 256.

[potamides]

Hello there, I absolutely love this terminal emulator, however for my usecase I'd need support for setting all colors in the 256 color table, do you have any interest in that? This would be useful for colorschemes like gruvbox which define a lot of colors outside the standard 4-bit color range. This pull request is my attempt to address this shortcoming. I followed the approach of alacritty here which beside the ansi and bright tables allows to set arbitrary colors from 16 to 256 in an indexed table. This would for example make it possible to specify the whole gruvbox colorscheme like this:

local config = {
  color_scheme = "Gruvbox Dark Medium",
  color_schemes = {
    ["Gruvbox Dark Medium"] = {
      foreground = "#ebdbb2",
      background = "#282828",
      cursor_bg = "#ebdbb2",
      cursor_border = "#ebdbb2",
      cursor_fg = "#282828",
      selection_bg = "#ebdbb2",
      selection_fg = "#282828",

      ansi = {"#282828","#cc241d","#98971a","#d79921","#458588","#b16286","#689d6a","#a89984"},
      brights = {"#928374","#fb4934","#b8bb26","#fabd2f","#83a598","#d3869b","#8ec07c","#ebdbb2"},
      indexed = {
        [24]  = "#076678", [66]  = "#458588", [72]  = "#689d6a", [88]  = "#9d0006", [96]  = "#8f3f71",
        [100] = "#79740e", [106] = "#98971a", [108] = "#8ec07c", [109] = "#83a598", [124] = "#cc241d",
        [130] = "#af3a03", [132] = "#b16286", [136] = "#b57614", [142] = "#b8bb26", [166] = "#d65d0e",
        [167] = "#fb4934", [172] = "#d79921", [175] = "#d3869b", [208] = "#fe8019", [214] = "#fabd2f",
        [223] = "#ebdbb2", [228] = "#f2e5bc", [229] = "#fbf1c7", [230] = "#f9f5d7", [234] = "#1d2021",
        [235] = "#282828", [236] = "#32302f", [237] = "#3c3836", [239] = "#504945", [241] = "#665c54",
        [243] = "#7c6f64", [244] = "#928374", [245] = "#928374", [246] = "#a89984", [248] = "#bdae93",
        [250] = "#d5c4a1"
      }
    }
  }
}

This pull request would fix #841.

[wez]

Thanks for this! #841 is a feature request to do this, so you're not alone!

[potamides]

Thanks for this! #841 is a feature request to do this, so you're not alone!

Nice to see, I'll link this issue then.

[wez]

Thanks!