Add a JS-backend only option to nim.cfg

[halloleo]

Mmmmh,
    
    
    @if js:
      define:release
    @end
    
    
    Run

doesn't seem to work for me.

Can I use a `.nims` file instead of `~/.config/nim/nim.cfg`? Is it then called 
`~/.config/nim/nim.nims`?