Add rescue option

Add rescue option to `ruboty-gen gem` command.

## Example

~~~bash
$ ruboty-gen gem some_plugin some_action --rescue
~~~

* generated code

~~~ruby
module Ruboty
  module SomePlugin
    module Actions
      class SomeAction < Ruboty::Actions::Base
        def call
          message.reply(some_action)
        end
      end

      def some_action
        "TODO: write your logic."
      rescue => e
        e.message
      end
    end
  end
end
~~~