Alllow setting winbled for notification windows

[stsewd]

windblend is a window option to allow transparency on windows. This is useful so notifications don't totally cover the text in the back. This could be a global option and could be set at these lines

nvim-notify/lua/notify/windows/init.lua

Lines 56 to 59 in 4743b32

	local win = api.nvim_open_win(notif_buf:buffer(), false, win_opts)
	vim.wo[win].winhl = "Normal:Normal,FloatBorder:" .. notif_buf.highlights.border
	vim.wo[win].wrap = false
	self.win_stages[win] = 2

[stsewd]

Fixed with #26 (comment)