Add applyAddon typings
Add applyAddon typings

Part of #1018
See #1130

xtermjs/xterm.js,xtermjs/xterm.js,akalipetis/xterm.js,Tyriar/xterm.js,akalipetis/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,akalipetis/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js