Change default log named ".%s.log" to ".qtile.log"

I freshly reinstalled qtile on my new laptop and I found a ``.%s.log`` file in my current directory.

A quick ``git grep`` shows that the culprit is https://github.com/qtile/qtile/blob/develop/libqtile/log_utils.py#L43 as pasted below:
```python
def init_log(log_level=logging.ERROR, logger='qtile', log_path='~/.%s.log'):
```

I propose to change the ``log_path`` default value to ``.qtile.log``.