Add option for file path prefix + add followlocation option to curl calls

[joeke]

This PR fixes an issue, and adds a new feature.

Fix:
Add CURLOPT_FOLLOWLOCATION to remote image curl calls to allow curl to follow redirects (e.g. 301) on the remote url.
(fixes issue #56)

Feature:
Add filePathPrefix option to enable a prefix for the output image path. This option can be used in the snippet call options parameter, for example:
[[pThumb?&input=`https://test.com/my-image.jpg`&options=`&w=300&h=300&filePathPrefix=[[*id]]`]]

Which will result in adding the resource id in the output file path (using resource ID 1 as an example):
assets/image-cache/1/my-image.jpg

This is a welcome addition when handling large amounts of images where the source images are all in the same folder, which results in the image-cache folder having a lot of items, and it also makes pthumb slower with checking existing files and generating thumbnails since it has to navigate through this large folder.

[joeke]

@matdave @theboxer Have you had the time to look into this?