add `brew cask open` command

thanks to @passcod for original implementation on his fork

refs #72

lib/cask/cli/open.rb

@@ -0,0 +1,16 @@
+module Cask::CLI::Open
+  def self.run(*cask_names)
+    cask_names.each do |cask_name|
+      begin
+        cask = Cask.load(cask_name)
+        system "open", cask.homepage
+      rescue CaskUnavailableError => e
+        onoe e
+      end
+    end
+  end
+
+  def self.help
+    "opens the homepage of the cask of the given name"
+  end
+end

test/cli/open_test.rb

@@ -0,0 +1,41 @@
+require 'test_helper'
+
+module RecordSystemCalls
+  def system(*command)
+    system_commands << command
+  end
+
+  def reset!
+    @system_commands = []
+  end
+
+  def system_commands
+    @system_commands ||= []
+  end
+end
+
+module Cask::CLI::Open
+  extend RecordSystemCalls
+end
+
+describe Cask::CLI::Open do
+  before do
+    Cask::CLI::Open.reset!
+  end
+
+  it 'opens the homepage for the specified cask' do
+    Cask::CLI::Open.run('alfred')
+    Cask::CLI::Open.system_commands.must_equal [
+      ['open', 'http://www.alfredapp.com/']
+    ]
+  end
+
+  it 'works for multiple casks' do
+    Cask::CLI::Open.run('alfred', 'adium')
+    Cask::CLI::Open.system_commands.must_equal [
+      ['open', 'http://www.alfredapp.com/'],
+      ['open', 'http://www.adium.im/']
+    ]
+  end
+end
+