Make --color attributes mergeable

So you can override the colors and still have the text attributes

    # Default colors and attributes
    fzf

    export FZF_DEFAULT_OPTS='--color hl:-1:underline,hl+:-1:underline:reverse'

    # Default colors with underline+reverse attributes
    fzf

    # Different colors with underline+reverse attributes
    fzf --color hl:176,hl+:177

Related: https://github.com/junegunn/fzf.vim/issues/1197#issuecomment-739804363