Add "bundle search" subcommand

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.

bin/bundler-search

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+# Search your bundle for the provided pattern
+#   Requires bundler 1.8+ for execution as a bundler subcommand.
+#   Examples:
+#     bundle search Kernal.warn
+#     bundle search current_user clearance
+#     bundle search "Change your password" clearance
+#
+# Arguments:
+#  1. What to search for
+#  2. Which gem names to search (defaults to all gems)
+
+pattern="$1"; shift
+ag "$pattern" $(bundle show --paths "$@")