Need SOUNDIO_VERSION_* in library header

Currently, this set of macros is placed in `config.h`:

```
#define SOUNDIO_VERSION_MAJOR 1
#define SOUNDIO_VERSION_MINOR 1
#define SOUNDIO_VERSION_PATCH 0
#define SOUNDIO_VERSION_STRING "1.1.0"
```

Applications consuming libsoundio cannot make use of these macros, because `config.h` is for use only by the libsoundio build itself. But these macros are extremely useful to applications, not least because they allow application code to adapt to different versions of the API. (For example, an application could potentially support both libsoundio API versions 1.1 and 2.0, despite these being mutually incompatible, by conditionalizing on the macros.)

I would say that these macros belong in the main `soundio.h` library header, so that they are part and parcel of the API definition.