support `brew cask edit`

lib/cask.rb

@@ -8,10 +8,12 @@ HOME_APPS = Pathname.new(File.expand_path("~/Applications"))
 class Cask; end
 
 require 'cask/cli'
+require 'cask/cli/edit'
 require 'cask/cli/install'
 require 'cask/cli/linkapps'
 require 'cask/cli/list'
 require 'cask/cli/search'
+require 'cask/exceptions'
 require 'plist/parser'
 
 class Cask

lib/cask/cli/edit.rb

@@ -0,0 +1,12 @@
+class Cask::CLI::Edit
+  def self.run(*arguments)
+    cask_name, *rest = *arguments
+    cask_path = Cask.path.join("#{cask_name}.rb")
+    raise CaskUnavailableError, cask_path.basename('.rb').to_s unless cask_path.file?
+    exec_editor cask_path
+  end
+
+  def self.help
+    "edits the cask of the given name"
+  end
+end

lib/cask/exceptions.rb

@@ -0,0 +1,10 @@
+class CaskUnavailableError < RuntimeError
+  attr_reader :name
+  def initialize name
+    @name = name
+  end
+
+  def to_s
+    "No available cask for #{name}"
+  end
+end