add Application.show_config[_json]

[minrk]

setting this value will skip Application start and show loaded config on stdout, instead.

if show_config_json is given, it will be json-formatted.

• tests

Applications can inherit the default flags from traitlets.config.application.default_flags,
but this can be triggered immediately with:

any-jupyter-command --Application.show_config=True

cc @rgbkrk

[rgbkrk]

Yay, this is great! I'll use this for smoke testing a Docker image before promoting to production.

[ankostis]

But why stop the app?
The option may just dump the configs in the logger (i.e. at INFO level), and continue normally.

This option is useful for every combination sub-commands and it was one of the first "extensions" I had to built my self.

[rgbkrk]

The reason I was hoping for something like this is to be able to run validation tests against Jupyter installations. I do want the application to stop after exiting.