Custom memory allocation hooks

[brunochampoux]

In order to use this library in a large-scale video game engine, we need to replace new/delete calls with custom memory allocation hooks. It would be nice if this was an official feature of the library!

There are many ways to handle this. However, in order to prevent the templating of all tinyxml2 classes, we have used an abstract class with Alloc/Free methods to override.

Feel free to discuss with us. We can prepare a pull request with our idea if you want.