Allow self-update via regular install/update commands

Today, upgrading RubyGems and Bundler requires "special" commands (gem update --system and bundle update --bundler, respectively).

It would improve the user experience if upgrading used the same commands to upgrade other gems. This would not only make it easier to remember how to upgrade, it would also get us more in line with other package managers, like pip (pip upgrade pip) and npm (npm install npm).

I propose we handle gem install gem and gem update gem, installing the latest version of RubyGems.

I also propose we allow bundle update bundler as a way to both 1) install the newest Bundler version (e.g. gem install bundler), and also 2) update the Bundler version used by the current Gemfile (e.g. bundle update --bundler).

Open questions:

1. Should we also handle gem install rubygems and gem update rubygems?
2. Should we also handle bundle update bundle?
3. Should we deprecate the existing update commands, or just leave them there for anyone who still remembers them?

Review the existing gem install, gem update, and bundle update command paths, including the special RubyGems and Bundler update commands named in the issue. Resolve the three open questions about accepted names, Bundler behavior, and deprecation. Done means regular install/update commands provide the proposed self-update behavior without breaking existing commands.