add PyInstaller script and hook (#166)

scripts/hook-gallery_dl.py

@@ -0,0 +1,9 @@
+# -*- coding: utf-8 -*-
+
+from gallery_dl import extractor, downloader, postprocessor
+
+hiddenimports = [
+    package.__name__ + "." + module
+    for package in (extractor, downloader, postprocessor)
+    for module in package.modules
+]