unicode is not support

[GHolk]

this library does not support unicode.
for example: require('qrcode-terminal').generate('αβγ') will generate QR code
which would be parsed to garbled text.

> require('qrcode-terminal').generate('αβγ', {small:true})
▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄
█ ▄▄▄▄▄ █▀▀ ▄ █ ▄▄▄▄▄ █
█ █   █ █▀▀▄▄ █ █   █ █
█ █▄▄▄█ █▀▄█▀▄█ █▄▄▄█ █
█▄▄▄▄▄▄▄█▄▀▄█▄█▄▄▄▄▄▄▄█
█ ▄▄▄ ▀▄▄ ▀█ ▀ ▀▄█▄▀ ▀█
█ ▄▀▀▀▄▄▄█▀ ▀ ▀ ▄███▀▄█
█▄▄██▄█▄▄▀ ▀█ ██ █▄▀▀▀█
█ ▄▄▄▄▄ █▄█ ▄█▄█▀ ▄▀▀ █
█ █   █ █  █ ▀ ▀█▄▀▄███
█ █▄▄▄█ █ ▀▀▀ ▀ ▄█▀████
█▄▄▄▄▄▄▄█▄███▄██▄██▄▄██