Add support for WebP format?

### Is your feature request related to a problem? Please describe.

GIL does not support images in the [WebP format](https://developers.google.com/speed/webp). In my experience, this format is becoming increasingly more common as a replacement for JPEG and PNG image formats. So at least read support for that format would be really nice. :)

### Describe the solution you'd like

There should be an I/O extension which can read WebP images (and possibly write such images, too).
Easiest way would probably be to use the existing [libwebp library](https://chromium.googlesource.com/webm/libwebp/) to do the reading and writing. Encoding/decoding of such images in one go is possible via [libwebp's API](https://developers.google.com/speed/webp/docs/api#simple_decoding_api).

### Questions

* Are there any intentions to add WebP support to Boost GIL?
* If yes: Is someone already working on such a feature? What is the best way to support the existing efforts?
* If no: What is the suggested alternative for handling WebP with Boost GIL? Would a pull requests that adds support for WebP be welcome?

Start by reviewing Boost.GIL's existing I/O extensions and the libwebp Simple Decoding API linked in the issue. Determine how read support should fit the current extension patterns, then establish tests for loading WebP images; writing support is optional and should be clarified before implementation.

Written by the indexing model from the issue text.