New command: brew unlinkapps

The name is pretty self explanatory, it unlinks all installed
applications found under `brew --prefix` from either `~/Applications` or
`/Applications`

Closes Homebrew/homebrew#22729.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/cmd/unlinkapps.rb

@@ -0,0 +1,28 @@
+# Unlinks any Applications (.app) found in installed prefixes from /Applications
+require 'keg'
+
+module Homebrew extend self
+
+  def unlinkapps
+    target_dir = ARGV.include?("--local") ? File.expand_path("~/Applications") : "/Applications"
+
+    unless File.exist? target_dir
+      opoo "#{target_dir} does not exist, stopping."
+      puts "Run `mkdir #{target_dir}` first."
+      exit 1
+    end
+
+    cellar_apps = Dir[target_dir + '/*.app'].select do |app|
+      if File.symlink?(app) && File.readlink(app).match(HOMEBREW_CELLAR)
+        File.readlink app
+      end
+    end
+
+    cellar_apps.each do |app|
+      puts "Unlinking #{app}"
+      system "unlink", app
+    end
+
+    puts "Finished unlinking from #{target_dir}" if cellar_apps
+  end
+end