Remove default values for -l and -S

The default values are now handled inside of `run_black`, since it uses
the function `read_pyproject_toml` from `black`, which will handle
setting default values if the keys are not defined in a `pyproject.toml`
file

src/darker/__main__.py

@@ -137,10 +137,11 @@ def main(argv: List[str] = None) -> None:
         logger.error(f"{ISORT_INSTRUCTION} to use the `--isort` option.")
         exit(1)
 
-    black_args = {
-        "line_length": args.line_length,
-        "skip_string_normalization": args.skip_string_normalization,
-    }
+    black_args = {}
+    if args.line_length:
+        black_args["line_length"] = args.line_length
+    if args.skip_string_normalization:
+        black_args["skip_string_normalization"] = args.skip_string_normalization
 
     paths = {Path(p) for p in args.src}
     format_edited_parts(paths, args.isort, black_args, args.config)

src/darker/command_line.py

@@ -60,15 +60,13 @@ def parse_command_line(argv: List[str]) -> Namespace:
         "--skip-string-normalization",
         action="store_true",
         dest="skip_string_normalization",
-        default=False,
         help="Don't normalize string quotes or prefixes",
     )
     parser.add_argument(
         "-l",
         "--line-length",
         type=int,
         dest="line_length",
-        default=88,
         help="How many characters per line to allow [default: 88]",
     )
     return parser.parse_args(argv)