Add a JS-backend only option to nim.cfg

[Araq]

Because the global nim.cfg contains
    
    
    @if release or danger:
      stacktrace:off
      excessiveStackTrace:off
      linetrace:off
      debugger:off
      line_dir:off
      opt:speed
      define:release
    @end
    
    
    Run

and is processed **before** your custom config file, when you set `-d:release` 
it's too late and you don't get the release mode.

A very annoying gotcha we're still trying to solve, somehow.