- Improve the generated config file output.
- Allow TRAITLETS_APPLICATION_RAISE_CONFIG_FILE_ERROR env to override
`Application.raise_config_file_errors`, so that config file errors can
result in exiting immediately.
- Avoid using root logger. If no application logger is registered, the
`'traitlets'` logger will be used instead of the root logger.
- Change/Validation arguments are now Bunch objects, allowing
attribute-access, in addition to dictionary access.
- Reduce number of common deprecation messages in certain cases.
- Ensure command-line options always have higher priority than config
files.
- Add bounds on numeric traits.
- Improves various error messages.