Add a JS-backend only option to nim.cfg

[halloleo]

I guess, a workaround can be to set the options myself (and overwrite what's 
set before):
    
    
    @if js:
      stacktrace:off
      excessiveStackTrace:off
      linetrace:off
      debugger:off
      line_dir:off
    @end
    
    
    Run