Add strict_prompts option

This adds a `strict_prompts` key to the `liftoffrc` and a
`--[no]-strict-prompts` command line flag.

Enabling this option causes Liftoff to skip the prompt for keys that
already have a default value.

defaults/liftoffrc

@@ -13,6 +13,7 @@ enable_static_analyzer: true
 indentation_level: 4
 use_tabs: false
 use_cocoapods: true
+strict_prompts: false
 
 run_script_phases:
   - todo.sh: Warn for TODO and FIXME comments

lib/liftoff/cli.rb

@@ -30,6 +30,10 @@ def global_options
           exit
         end
 
+        opts.on('--[no-]strict-prompts', 'Enable/Disable strict prompts') do |strict_prompts|
+          @options[:strict_prompts] = strict_prompts
+        end
+
         opts.on('--[no-]cocoapods', 'Enable/Disable Cocoapods') do |use_cocoapods|
           @options[:use_cocoapods] = use_cocoapods
         end

lib/liftoff/option_fetcher.rb

@@ -14,13 +14,20 @@ def fetch_options
     private
 
     def fetch_option_for(attribute, prompt)
-      value = ask("#{prompt}? ") { |q| q.default = @configuration.public_send(attribute) }
-      @configuration.public_send("#{attribute}=", value)
+      default = @configuration.public_send(attribute)
+      unless skip_prompt?(default)
+        value = ask("#{prompt}? ") { |q| q.default = default }
+        @configuration.public_send("#{attribute}=", value)
+      end
     rescue EOFError
       puts
       fetch_option_for(attribute, prompt)
     rescue Interrupt
       exit 1
     end
+
+    def skip_prompt?(default)
+      default && @configuration.strict_prompts
+    end
   end
 end

lib/liftoff/project_configuration.rb

@@ -13,7 +13,8 @@ class ProjectConfiguration
       :application_target_groups,
       :unit_test_target_groups,
       :use_cocoapods,
-      :run_script_phases
+      :run_script_phases,
+      :strict_prompts
 
     attr_writer :author,
       :company_identifier,