We can install RubyGems plugin by "gem install XXX". The installed
plugin is used from the NEXT "gem ...".

For example, "gem install gem-src kaminari" doesn't use gem-src plugin
for kaminari. "gem install gem-src && gem install kaminari" uses
gem-src plugin for kaminari.

How about loading a plugin immediately when the plugin is installed?
If this proposal is implemented, "gem install gem-src kaminari" works
like "gem install gem-src && gem install kaminari".

https://github.com/rubygems/rubygems/commit/4917d96f4c