Add resize_mode option to watermark filter

[tobymarsden]

I needed the ability to resize a watermark to the full size of the source image, ignoring the watermark's aspect ratio. This PR allows you to do this:

http://localhost:8000/unsafe/0x0:800x500/fit-in/500x0/filters:watermark(https://ucarecdn.com/3ebe2903-f913-4293-9228-2ec723dbb175/,center,center,0,100,100,force)/https://ucarecdn.com/638d660f-3759-494d-9ae3-08eddf68bd49/

Before this change:

After this change:

[cshum]

Thanks for your PR! Would you mind adding a line of test image uri similar to this？ https://github.com/cshum/imagor/blob/master/vips/processor_test.go#L159