Traitlets

- 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.

Partially revert a change in 4.1 that prevented IPython's command-line
options from taking priority over config files.

Demotes warning about unused arguments in `HasTraits.__init__`
introduced in 4.2.0 to DeprecationWarning.

- `JSONFileConfigLoader` can be used
as a context manager for updating configuration.
- If a value in config does not map onto a configurable trait, a
message is displayed that the value will have no effect.
- Unused arguments are passed to `super()` in `HasTraits.__init__`,
improving support for multiple inheritance.
- Various bugfixes and improvements in the new API introduced in 4.1.
- Application subclasses may specify `raise_config_file_errors = True`
to exit on failure to load config files, instead of the default of
logging the failures.