Add support for job folders

[NfNitLoop]

Our Jenkins server groups lots of builds by folders, but jenkins jobs only shows the 2 top-level jobs we've defined.

When using Jenkinsfiles I don't think we even have the option to specify those builds in the top level of Jenkins, so jenkins-cli isn't really usable in that case.

[NfNitLoop]

Aha, I see. I can sortof work around this issue with a config file that looks like this:

# ~/.jenkins-cli

[DEFAULT]
host=https://jenkins.example.com/
username=foo
password=bar

[my_project]
host=https://jenkins.example.com/job/MyOrgGitHubRepos/job/my_project/

Then I can do jenkins -e my_project jobs. If that's the intended way to use that, it should be documented!

But I still maintain it'd be nice to have a way to work w/ jobs in folders w/o resorting to the above config hacks.