Add option to lossy or lossless with value

[solracsf]

Add two simple inputs on the plugin Admin:

• choice between lossy or lossless
• if lossy, specify the compression factor for RGB channels between 0 and 100

Then pass those params to exec("$cwebp -quiet $imagePath -o $webpPath"); like

exec("$cwebp -quiet -lossless $imagePath -o $webpPath");

Doc: https://developers.google.com/speed/webp/docs/cwebp

[jissereitsma]

Unfortunately the PHP function for converting WebP does not have this option (http://php.net/manual/en/function.imagewebp.php), only the CLI option has. So instead of configuring bin/webp as your binary, why not simply set bin/webp -lossless?