Clean out/* on build in case src/* files were deleted
Clean out/* on build in case src/* files were deleted

If files in src/ were compiled and then deleted they will hang around in out/
until deleted. This can potentially cause difficult to diagnose build/test
errors. This commit removes all files from out/ but keeps the folder to
prevent editor file watchers from seeing the folder was deleted and shifting
the file explorer around.

Fixed #381

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