Correct IBufferLine.getCell API
Correct IBufferLine.getCell API

Fixes #2182

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