Hash generation from relative path instead of absolute

[ghost]

I think that generating hashes from relative path instead of absolute path would allow more usages of this library.

For example, I am working on a website made with Grav CMS, and there's lots of images generation, so first page loads takes lots times (scaling, transforming images etc..). Before deploying in production, I want to ensure that every image has been generated and compiled (to not make the prod verrrry slooww), it does execute php -S ... and crawl all the pages with wget -r ... then doing a rsync the images/ directory from local to prod.

But because the absolute path is used, the pre-generated images on local won't be used by prod. Prod having as base path something like /opt/sflx/live/ and the local have something like /home/dctremblay/Containers/SFLxNG/build/live. If relative path is used instead of absolute, this trick would work. The commit introduces this fix.

If you have further questions do not hesitate, thank you very much and have a great day !