Minor change to prevent gcc -Weffc++ warnings. 

Hi c42f, 

I like your tool, thanks for that!
I did not know that there is such a small implementation to use std::string with the s/printf syntax, which I prefer over std::stringstream. 

We use in our project -Weffc++ and usually build warnings free. I thought instead of branching here permanently, I send you the two lines to prevent the warnings as pull request, perhaps you want to apply it.

Cheers
Uli