feat: aerich.Command support async with syntax (#427)

* feat: `aerich.Command` support `async with` syntax

* docs: update readme

tests/test_command.py

@@ -0,0 +1,11 @@
+from aerich import Command
+from conftest import tortoise_orm
+
+
+async def test_command(mocker):
+    mocker.patch("os.listdir", return_value=[])
+    async with Command(tortoise_orm) as command:
+        history = await command.history()
+        heads = await command.heads()
+    assert history == []
+    assert heads == []