webp thumbnails

I would like to add support for .webp thumbnail generation from source images.

https://github.com/haraldk/TwelveMonkeys

<dependency>
	<groupId>com.twelvemonkeys.imageio</groupId>
	<artifactId>imageio-webp</artifactId>
	<version>3.8.1</version>
</dependency>

Please note the instructions in https://github.com/haraldk/TwelveMonkeys#deploying-the-plugins-in-a-web-app