Add HDR Tone Mapping Function

This enables `af::Window::image()` to render HDR images
that are loaded using `af::loadImage`

Tentative API looks as follows

```
af::array toneMap(const array& hdrImage, const ToneMapFunc mapFunc); // C++ API
af_err af_tone_map(af_array* out, const af_array hdrImage, const af_tone_map_t mp); // C API
```

where `ToneMapFunc`/`af_tone_map_t` will be an enum with values such as follows
- Linear
- Logarithmic

Please feel free to add any tone mapping methods (some type of text book equations that we can use in an Open Source library) to the above list. I am still exploring on what kind of tone mapping algorithms are available.