xterm.js integration

[milahu]

https://github.com/xtermjs/xterm.js is a popular terminal emulator

with LocalEchoController we can add custom interpreters

see also https://github.com/RangerMauve/xterm-js-shell/blob/master/index.js

benefit of bash-emulator: its faster for plain text (no ansi escapes)