Allow rdoc to be an optional dependency?

While an rdoc integration into irb has definite advantages, there are circumstances where the rdoc integration is more of a hinderance, e.g. wenn used as a console in production-like environment.

This also extends to the rdocs dependencies - if e.g. the project in question does not otherwise use ERB, we will always need to install it in the production environments anyway. And then be confronted with handling security issues for ERB, even if they aren't really pertinent to the project in question.

I'm not aware of an easy way to mark dependencies as optional with the current gemfile specification, so this issue might need to be put on the back burner until such a time that this can be specified in the gemfile. :)

Start by reviewing the rdoc integration in IRB and how its dependencies are specified in the Gemfile. Define how rdoc and ERB could be optional while preserving the integration for users who need it; done means IRB can be used without installing those dependencies when the integration is not required.