Bundler 1.8+ added support for git-style subcommands. Any scripts
starting with `bundler-` in your path are executable as bundler
subcommands.
This adds `search` as a subcommand that uses `ag` to search for a string
among all gems in your bundle (default) or optionally a specific gem.
I've found this useful for finding the source of puzzling deprecations,
finding what gem provides a method, and other various things.
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