Move shell methods to a general helper

The helpers that interact with the user will need to be called from
outside of the `Cli` namespace in the future. I need classes loaded
before the `Cli` classes to be able to require a module like the `Shell`
one to access those methods.

I've added `module_function` to the module. This makes it possible to
call the methods on the module directly, without having to include the
module.

```
Shell.ask_for_input
Shell.required_input "Your name?"
Shell.yes_or_no "Cake?"
```

lib/mono.rb

@@ -39,6 +39,7 @@ def message
   end
 end
 
+require "mono/shell"
 require "mono/version"
 require "mono/version_object"
 require "mono/version_promoter"

lib/mono/cli.rb

@@ -6,40 +6,8 @@
 
 module Mono
   module Cli
-    module Helpers
-      def ask_for_input
-        value = $stdin.gets
-        value ? value.chomp : ""
-      rescue Interrupt
-        puts "\nExiting..."
-        exit 1
-      end
-
-      def required_input(prompt)
-        loop do
-          print prompt
-          value = ask_for_input
-          return value unless value.empty?
-        end
-      end
-
-      def yes_or_no(prompt, options = {})
-        loop do
-          print prompt
-          input = ask_for_input.strip
-          input = options[:default] if input.empty? && options[:default]
-          case input
-          when "y", "Y", "yes"
-            return true
-          when "n", "N", "no"
-            return false
-          end
-        end
-      end
-    end
-
     class Base
-      include Helpers
+      include Shell
       include Command::Helper
 
       def initialize(options = {})

lib/mono/cli/init.rb

@@ -3,7 +3,7 @@
 module Mono
   module Cli
     class Init
-      include Helpers
+      include Shell
 
       def execute
         config = {}

lib/mono/shell.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+module Mono
+  module Shell
+    module_function
+
+    def ask_for_input
+      value = $stdin.gets
+      value ? value.chomp : ""
+    rescue Interrupt
+      puts "\nExiting..."
+      exit 1
+    end
+
+    def required_input(prompt)
+      loop do
+        print prompt
+        value = ask_for_input
+        return value unless value.empty?
+      end
+    end
+
+    def yes_or_no(prompt, options = {})
+      loop do
+        print prompt
+        input = ask_for_input.strip
+        input = options[:default] if input.empty? && options[:default]
+        case input
+        when "y", "Y", "yes"
+          return true
+        when "n", "N", "no"
+          return false
+        end
+      end
+    end
+  end
+end