Implement format command

Closes #4 (closed)

dan/datasets/format/__init__.py

+# -*- coding: utf-8 -*-
+"""
+Format datasets for training.
+"""
+
+from pathlib import Path
+
+from dan.datasets.format.atr import run
+
+
+def add_format_parser(subcommands) -> None:
+    parser = subcommands.add_parser(
+        "format",
+        description=__doc__,
+        help=__doc__,
+    )
+    parser.add_argument(
+        "--dataset",
+        type=Path,
+        help="Path to the exported dataset.",
+        required=True,
+    )
+    parser.add_argument(
+        "--image-format",
+        type=str,
+        help="Format under which the images were saved.",
+        required=True,
+    )
+    parser.add_argument(
+        "--keep-spaces",
+        action="store_true",
+        help="Do not remove spaces in transcriptions.",
+    )
+
+    parser.set_defaults(func=run)