Handle blob in attach Addon
Handle blob in attach Addon

I have added blob handler for the attach Addon.
This allows to handle Docker with ws (works on Docker API v1.35)
I have also added a callback to handle asynchronous FileReader.
xtermjs/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,akalipetis/xterm.js,Tyriar/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,akalipetis/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js,sourcelair/xterm.js