Bundler: additional integrations

[boardfish]

bundle gem is currently helpful in creating some of the things most gems should start out with, such as CI config. But I'd like to suggest a few more features that I think should be part of bundle gem before I start working on them:

• A pattern for top-level configuration of a gem, e.g. being able to call YourGem.config to have a Struct you can query and modify to set options. Optionally, you could also depend on ActiveSupport if you're not shy of that and use its OrderedOptions class to provide this in the same way Rails does.
• Exposure of this config to a Rails app via an engine initializer, as is done here. It's of note that at present, ViewComponent is configured via mattr_accessors on the class, rather than by its own config object, so those config options need to be manually mirrored to Rails.application.config.view_component.
• Providing a means of generating a documentation site that can be shared, for example, on GitHub Pages
• Providing a means of generating a changelog, e.g. with git-chglog
• CI configuration for pushing new versions of gems to registries
• CI configuration for running RuboCop, SimpleCov and RSpec if not already provided

Some of these things are opinionated and may be optional, but it would be better to make as many defaults as possible so that we can guide folks towards making better gems that provide a good framework for themselves to grow.