Add canvas.tobwstring

[Sainan]

A bit of a compromise on colour, but at least portable.

local canvas = require "canvas"
local c = canvas.qrcode("Hello from Pluto!", { fg = 0xffffff, bg = 0x000000 })
print(c:tobwstring(0xffffff))
--> █▀▀▀▀▀█ ▄ █▄▄ █▀▀▀▀▀█
--> █ ███ █ ▄█▄▄█ █ ███ █
--> █ ▀▀▀ █ ▄▀ █▄ █ ▀▀▀ █
--> ▀▀▀▀▀▀▀ ▀▄▀▄█ ▀▀▀▀▀▀▀
--> ▀▀███ ▀███ █▀▀ ▀▄█▄▀▄
--> █▄▀▄▀█▀█▀▀ ▀█▄▀▄▄██▀ 
--> ▀▀▀▀▀ ▀ █▀█▀ ▄█▄▄▄  ▀
--> █▀▀▀▀▀█ ▀  ▀██▀▄ ▄██ 
--> █ ███ █ █▄ ▄█▄██▄▄ ▄ 
--> █ ▀▀▀ █ █▄ ▀█▀ ▀▄▄█  
--> ▀▀▀▀▀▀▀ ▀   ▀ ▀ ▀  ▀