feat(init): add default_color option to colortils setup

[soraliu]

This pull request adds the default_color option to the colortils setup in both the readme.md file and colortils/init.lua.

The default_color option allows users to set a default color value when no color is found. This enhancement provides more flexibility and customization to the colortils plugin.

Use Cases:

-- Setting default_color to a specific color
require("colortils").setup({
    default_color = "#FF0000",
    -- Other settings
})

BTW, correct a typo in the README.md

Feel free to review and test this enhancement. Thank you!

[max397574]

Thank you