nestlog -- logging with automatic indenting according to call graph

Nestlog is a lightweight header-only library for console logging.

Examples

```
/* examples/ex1/ex1.cpp */

#include "nestlog/scope.hpp"

using namespace xo;

void A(int x) {
    XO_SCOPE(log)  // i.e. xo::scope log("A");

    log(":x ", x);
}

int
main(int argc, char ** argv) {
    A(66);
}
```

output: +A :x 66 -A