Prevent mousedown event default selection behavior
Prevent mousedown event default selection behavior

Fixes #698

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