Create Context Manager for LogWriter

By implementing the [necessary methods](https://book.pythontips.com/en/latest/context_managers.html) for LogWriter, it can support the Context Manager syntax:

```python
writer = LogWriter.with_datetime(folder="./", threshold=10)

# Do stuff

writer.flush()
```

would become

```python
with LogWriter.with_datetime(folder="./", threshold=10) as writer:
    # Do stuff
```



