extra field filled at runtime

@mfrata is it possible to use an extra field filled at runtime? Example:

```python
$ export LOG_EXTRA_FIELDS='{"client_id": "%(client_id)s"}'

>>> from lodge import log
>>> log.error("some error", client_id=1)
```

or does it make sense?

_Originally posted by @brenocosta0901 in https://github.com/NeowayLabs/lodge/pull/1#discussion_r599205347_