trans png, jpeg image to webp
#usage ./image2webp -src=srcpath -dest=destpath -mode=lossless_rgba|lossless_rgb|lossless_gray|rgba90|rgba80|rgba70|rgb90|rgb80|rgb70|gray90|gray80|gray70
#use github.com/chai2010/webp this is a wapper of libwebp and just trans image to webp