feat: allow using bg and style on syntax and built_in colorscheme properties

[danielefongo]

With this PR I allow the user to insert style tables on colorscheme.syntax and colorscheme.built_in.

Old:

local syntax = {
  statement = "#ffffff",
  ["function"] = "#ffffff",
  variable = "#ffffff",
  include = "#ffffff",
  ...
}

New:

local syntax = {
  statement = { fg = "#ffffff", style = "bold" },
  ["function"] = { fg = "#ffffff", style = "italic" },
  variable = { fg = "#ffffff", bg = "red" },
  include = "#ffffff",
  ...
}

It is also retrocompatible with previous configurations.

[danielefongo]

Is this project dead? :C