Ecs-logging

- Added collecting `LogRecord.exc_info` into `error.*` fields
automatically for `StdlibFormatter` ([16](https://github.com/elastic/ecs-logging-python/pull/16))
- Added collecting process and thread info from `LogRecord` into `process.*` fields
automatically for `StdlibFormatter` ([16](https://github.com/elastic/ecs-logging-python/pull/16))
- Added `exclude_fields` parameter to `StdlibFormatter` to
exclude fields from being formatted to JSON ([16](https://github.com/elastic/ecs-logging-python/pull/16))
- Added `stack_trace_limit` parameter to `StdlibFormatter`
to control the number of stack trace frames being
formatted in `error.stack_trace` ([16](https://github.com/elastic/ecs-logging-python/pull/16))

Thanks to community contributor Jon Moore ([comcast-jonm](https://github.com/comcast-jonm))
for their contributions to this release.

- Added support for using `log(..., extra={...})` on standard library
loggers to use extended and custom fields ([8](https://github.com/elastic/ecs-logging-python/pull/8))

- Added `StdlibFormatter` for use with the standard library `logging` module
- Added `StructlogFormatter` for use with the `structlog` package