Generate manpage from configuration options

In https://github.com/jupyter/jupyter_server/issues/320, @SnarkBoojum submitted a manpage for Jupyter Server that largely encoded the results from `--help`. We realized that it would make more sense to have an automatically generated manpage at the traitlet level from the configurable traits. This issue is about adding a `--generate-manpage` option, or perhaps a separate manpage generation command, that does something similar to `-h`, but generates the output as a manpage.