Add support for webp images as a valid image Asset type

[khalwat]

webp is a really nice image format that has the weight of Google behind it; and it delivers really nice optimization of images in terms of file size.

While not all browsers support it yet, leaving it up to the developer to decide if it should be allowed would be useful. Here are some webp images from Google for a comparison:

https://developers.google.com/speed/webp/gallery1

Additionally, what many websites are doing is having the webserver itself handle the delivery of webp images based on the Accept header:

Nginx: https://github.com/uhop/grunt-tight-sprite/wiki/Recipe:-serve-WebP-with-nginx-conditionally

Apache: https://gist.github.com/seeekr/2415528

YouTube and other major sites do this; if you visit their site using Chrome, you get webp images. If you visit their site using a browser that doesn't support them, you get jpg or png

As long as Craft supports webp as an allowable image type, we'd be able to deliver this functionality in, even automatically pre-created the webp versions of the images if #1544 is implemented.

[khalwat]

aelvan/Imager-Craft#90

[khalwat]

Adding for completeness:

https://www.vaersaagod.no/en/using-webp-images-in-craft-with-imager

[andris-sevcenko]

Improved the support for this with e4ba1e3