fix: support 'time' and 'levelname' in default config

[KyleKing]

For: https://github.com/marselester/json-log-formatter/blob/f8d79df935f82b068611e6e01ff1e0577b92ce19/json_log_formatter/__init__.py#L184

And resolve:

Traceback (most recent call last):
  File "/Users/kyleking/.local/bin/tail-jsonl", line 8, in <module>
    sys.exit(start())
             ^^^^^^^
  File "<@beartype(tail_jsonl.scripts.start) at 0x1064cfc40>", line 11, in start
  File "/Users/kyleking/.local/pipx/venvs/tail-jsonl/lib/python3.12/site-packages/tail_jsonl/scripts.py", line 41, in start
    config = _load_config(options.config_path)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<@beartype(tail_jsonl.scripts._load_config) at 0x104eb6700>", line 35, in _load_config
  File "/Users/kyleking/.local/pipx/venvs/tail-jsonl/lib/python3.12/site-packages/tail_jsonl/scripts.py", line 23, in _load_config
    user_config = tomllib.loads(Path(config_path).read_text(encoding='utf-8'))
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/[email protected]/3.12.6/Frameworks/Python.framework/Versions/3.12/lib/python3.12/pathlib.py", line 1027, in read_text
    with self.open(mode='r', encoding=encoding, errors=errors) as f:
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/[email protected]/3.12.6/Frameworks/Python.framework/Versions/3.12/lib/python3.12/pathlib.py", line 1013, in open
    return io.open(self, mode, buffering, encoding, errors, newline)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '~/.config/tail-jsonl/config.toml'

Generally document how the configuration file is found (only manually)