feat: Enable --depth flag (still does nothing)

tests/test_args.py

@@ -0,0 +1,7 @@
+import subprocess
+
+from .fixtures import *
+
+def test_depth_flag_is_accepted(tmp_path, process):
+    arg_process = subprocess.run(["archivebox", "add", "https://example.com", "--depth=0"], capture_output=True)
+    assert 'unrecognized arguments: --depth' not in arg_process.stderr.decode('utf-8')