Add elisp hook to export menu

[ofosos]

Add elisp hook to export menu. I.e. when you export something you can place an elisp snippet in a custom variable that then get's executed with the new epub filename as a parameter.

[pank]

I think similar functions are already built in? I'd guess mostly anything can be done via org-export-filter-parse-tree-functions, org-export-filter-*-functions, org-export-before-processing-hook, and org-export-before-parsing-hook.