Add option to preload extensions

By creating a file called "preload.py" in an extension folder and declaring a preload(parser) method, we can add extra command-line args for an extension.

modules/extensions.py

@@ -1,12 +1,12 @@
 import os
 import sys
 import traceback
+from importlib.machinery import SourceFileLoader
 
 import git
 
 from modules import paths, shared
 
-
 extensions = []
 extensions_dir = os.path.join(paths.script_path, "extensions")
 
@@ -84,3 +84,24 @@ def list_extensions():
 
         extension = Extension(name=dirname, path=path, enabled=dirname not in shared.opts.disabled_extensions)
         extensions.append(extension)
+
+
+def preload_extensions(parser):
+    if not os.path.isdir(extensions_dir):
+        return
+
+    for dirname in sorted(os.listdir(extensions_dir)):
+        path = os.path.join(extensions_dir, dirname)
+        if not os.path.isdir(path):
+            continue
+        for file in os.listdir(path):
+            if "preload.py" in file:
+                full_file = os.path.join(path, file)
+                print(f"Got preload file: {full_file}")
+
+                try:
+                    ext = SourceFileLoader("preload", full_file).load_module()
+                    parser = ext.preload(parser)
+                except Exception as e:
+                    print(f"Exception preloading script: {e}")
+    return parser