title: Add a newline every set amount of letters in std::string
body: <p>I'm working with a few long strings inside a C++ program (up to 65535 letters).
  What am looking for is a way to add a new line every set amount of letters with an function like this:</p>
  
  <pre><code>addNewLinesToString(std::string* string, u8 lettersBetween newline);
  </code></pre>
  
  <p>Which would work like this:</p>
  
  <pre><code>string test = "Testing1234567";
  addNewLinesToString(&amp;test, 7); //test == "Testing\n1234567\n"
  </code></pre>
  
  <p>I have yet to find such a function that only used the standard class library (C and/or C++)</p>
  
  <p>If somebody has a simple solution for this would be great :)
  Thanks.</p>