allow fg, bg, attributes to be set for all colors in color_config

[fdncred]

this change allows all colors in the color_config section of the config.nu to have foreground, background, and color attributes. the foreground and background are written in hex colors like #rrggbb and the attributes are a concatenated string with the following values and meaning.

    for ch in color_attr.to_lowercase().chars() {
        match ch {
            'l' => blink = true,
            'b' => bold = true,
            'd' => dimmed = true,
            'h' => hidden = true,
            'i' => italic = true,
            'r' => reverse = true,
            's' => strikethrough = true,
            'u' => underline = true,
            'n' => (),
            _ => (),
        }
    }

the config.nu color_config section could look like this. comments are allowed, quotes around the hex colors and attributes are required.

  color_config: {
    separator: "#88b719" #yd
    leading_trailing_space_bg: white
    # header: cb
    header: {
      fg: "#B01455",
      bg: "#ffb900",
      attr: bli
    }
    date: "#75507B" #pu
    filesize: "#729fcf" #ub
    # row_index: yb
    row_index: {
      fg: "#e50914"
    }

so the bli in the header attr section means bold blink italic.
let the crazy colors begin. :)

[kubouch]

@fdncred Can you also update nushell/nushell#4300 with the new config?

[fdncred]

@fdncred Can you also update nushell/nushell#4300 with the new config?

ya, once I land this I will. I'd like to get some input on the section I talked about above.