Add support for WEBP .webp files

[John Smith]

Adding support for .webp image files would be useful as these offer great savings on disk space and are very lightweight on download and are supported in all modern browsers and you can use the <picture> element to provide fall bask .jpg. 

Since AtoM appears to use exec() calls to magick to generate thumbnails etc. - not the PHP API for magick as it should - this should be quite straight forward to implement? 

Similarly the watermarking with magick could be improved using text overlays rather than using .png files which make the viewing copies look yeuch! For example the magick command (extract from a bash script I use)

magick "$output_jpeg" \

\( -background none \

-fill "rgba(255,255,255,0.2)" \

-font Arial \

-pointsize 96 \

label:"Example Archives" \

-rotate 45 \) \

-gravity center \

-composite "$output_jpeg"

Produces this type of watermark

Just a suggestion :-)