Plugin system for templates

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Following on https://github.com/cowboyd/jenkins.rb/issues/31#issuecomment-3720385

What I propose initially is the ability to supply Ruby files that will be required at runtime by Jenkins. Users can then define and load their own templating code based on the existing Job::ConfigBuilder class.

Scenario: Create job with steps from custom template (jenkins create -r my_sweet_config_builder.rb --template my_template)

This is a similar pattern to, say, RSpec where you can load and use custom formatters at runtime.