[postprocessor:exec] improve (#421, #413)

- add 'final' option
- include job status in pp finalization
- improve and extend documentation

gallery_dl/postprocessor/zip.py

@@ -59,7 +59,7 @@ class ZipPP(PostProcessor):
         with zipfile.ZipFile(*self.args) as zfile:
             self._write(pathfmt, zfile)
 
-    def finalize(self):
+    def run_final(self, pathfmt, status):
         if self.zfile:
             self.zfile.close()