feature request: export version to header

Something like:

```cpp
// coordgen/CoordgenConfig.hpp
#define COORDGEN_MAJOR 3
#define COORDGEN_MINOR 0
#define COORDGEN_PATCH 2
#define COORDGEN_VERSION "3.0.2"
```

which enables

```cpp
#include <iostream>
#include <coordgen/CoordgenConfig.hpp>

int main() {
  std::cout << COORDGEN_VERSION << std::endl;
}
```