strformat using a run-time defined format string

[woggy]

In Python I can do something like this 
    
    
    f = "{:6e}"
    print(f.format(5))  # '5.000000e+00'
    
    
    Run

I can't figure out how to do something like this in Nim. As far as I can tell 
this is not possible using `strformat`. Can someone point me in the right 
direction?

Thanks.