inherit base aliases/flags from traitlets Application

[minrk]

gets --show-config and --show-config-json options for all Applications when using traitlets 5

[minrk]

This gets, for example:

$ jupyter lab --show-config
...
Loaded config files:
  /Users/minrk/.jupyter/jupyter_server_config.py
  /Users/minrk/.jupyter/jupyter_server_config.json

LabApp
  .open_browser = False
NotebookApp
  .nbserver_extensions = {'myext': False}
  .open_browser = False
NotebookManager
  .notebook_dir = '/Users/minrk/dev/mine/notebooks'
ServerApp
  .default_url = '/lab'
  .file_url_prefix = '/lab/tree'
  .jpserver_extensions = <LazyConfigValue value={'jupyterlab': True, 'dask_labextension': True, 'ipyparallel': True, 'jupyter_server_mathjax': True, 'jupyter_server_proxy': True, 'jupyterlab_code_formatter': True, 'nbclassic': True, 'nbdime': True}>
  .notebook_dir = '/Users/minrk/dev/mine/notebooks'
  .open_browser = False
  .password = 'sha1:...'
  .root_dir = '/Users/minrk/dev/mine/notebooks'

[blink1073]

Thank you!