Begin work on standalone version

Now the plugin ZIP file (DeDRM_plugin.zip) can be run with a normal
Python interpreter as if it were a Python file (try
`python3 DeDRM_plugin.zip --help`). This way I can begin building a
standalone version (that can run without Calibre) without having to
duplicate a ton of code.

DeDRM_plugin/__main__.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+# __main__.py for DeDRM_plugin
+# (CLI interface without Calibre)
+# Copyright © 2021 NoDRM
+
+__license__   = 'GPL v3'
+__docformat__ = 'restructuredtext en'
+
+# For revision history see __init__.py
+
+"""
+Run DeDRM plugin without Calibre.
+"""
+
+# Import __init__.py from the standalone folder so we can have all the 
+# standalone / non-Calibre code in that subfolder.
+
+import standalone.__init__ as mdata
+import sys
+
+mdata.main(sys.argv)