First attempt to add xelatex for issue #2

This is my first attempt to add xelatex, additionally to the xetex option. The fix is not optimal, because, whether it compiles the file at all or compiles the file to pdf after that, happens quite randomly. I'm new to Brackets plugin development, so I don't know how to debug this stuff. It seems that it quite often ends in an infinite loop or something similar, because after the 'Please wait... ' message either nothing or an empty line is shown. Rarely the output of xetex command is shown with all the debug details. Perhaps you can spot the problem?