In org-publish-projects, it would be good if the preparation-function is
executed before getting base files using org-publish-get-base-files.

In one of my use cases, my preparation-function generates some source
files which are then to be published by the publishing-function. But,
since the base filenames are collected before the preparation-function
is executed, the publishing-function does not see the generated source
files.

Right now, I'm calling my preparation-function manually before calling
org-publish-project, but it would be nice if I didn't have to do it this
way.

Thanks,
Arun Isaac.