Use RuboCop Performance for development

This commit uses RuboCop Performance for development and
fixes the following offenses.

```console
% bundle exec rake
bundle exec rubocop
Inspecting 116 files
...C................................................................................................................

Offenses:

tasks/cops_documentation.rake:60:15: C: Performance/UnfreezeString: Use
unary plus to get an unfrozen string literal.
    content = "\n".dup
              ^^^^^^^^
tasks/cops_documentation.rake:67:15: C: Performance/UnfreezeString: Use
unary plus to get an unfrozen string literal.
    content = "\n".dup
              ^^^^^^^^
tasks/cops_documentation.rake:74:15: C: Performance/UnfreezeString: Use
unary plus to get an unfrozen string literal.
    content = "#### #{title}\n".dup
              ^^^^^^^^^^^^^^^^^^^^^
tasks/cops_documentation.rake:80:15: C: Performance/UnfreezeString: Use
unary plus to get an unfrozen string literal.
    content = "```ruby\n".dup
              ^^^^^^^^^^^^^^^
tasks/cops_documentation.rake:171:15: C: Performance/UnfreezeString: Use
unary plus to get an unfrozen string literal.
    content = "# #{department}\n".dup
              ^^^^^^^^^^^^^^^^^^^^^^^
tasks/cops_documentation.rake:209:15: C: Performance/UnfreezeString: Use
unary plus to get an unfrozen string literal.
    content = "#### Department [#{type_title}](#{filename})\n\n".dup
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
tasks/cops_documentation.rake:222:15: C: Performance/UnfreezeString: Use
unary plus to get an unfrozen string literal.
    content = "<!-- START_COP_LIST -->\n".dup
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

116 files inspected, 7 offenses detected
rake aborted!
```

Author: koic

.rubocop.yml

@@ -3,6 +3,7 @@
 inherit_from: .rubocop_todo.yml
 require:
   - rubocop/cop/internal_affairs
+  - rubocop-performance
   - rubocop-rspec
 
 AllCops:

Gemfile

@@ -9,5 +9,6 @@ gemspec
 gem 'rake'
 gem 'rspec'
 gem 'rubocop', github: 'rubocop-hq/rubocop'
+gem 'rubocop-performance', '~> 1.3.0'
 gem 'rubocop-rspec', '~> 1.29.0'
 gem 'yard', '~> 0.9'

tasks/cops_documentation.rake

@@ -57,27 +57,27 @@ task generate_cops_documentation: :yard_for_generate_documentation do
   # rubocop:enable Metrics/MethodLength
 
   def h2(title)
-    content = "\n".dup
+    content = +"\n"
     content << "## #{title}\n"
     content << "\n"
     content
   end
 
   def h3(title)
-    content = "\n".dup
+    content = +"\n"
     content << "### #{title}\n"
     content << "\n"
     content
   end
 
   def h4(title)
-    content = "#### #{title}\n".dup
+    content = +"#### #{title}\n"
     content << "\n"
     content
   end
 
   def code_example(ruby_code)
-    content = "```ruby\n".dup
+    content = +"```ruby\n"
     content << ruby_code.text
                .gsub('@good', '# good').gsub('@bad', '# bad').strip
     content << "\n```\n"
@@ -168,7 +168,7 @@ task generate_cops_documentation: :yard_for_generate_documentation do
     end
     return if selected_cops.empty?
 
-    content = "# #{department}\n".dup
+    content = +"# #{department}\n"
     selected_cops.each do |cop|
       content << print_cop_with_doc(cop, config)
     end
@@ -206,7 +206,7 @@ task generate_cops_documentation: :yard_for_generate_documentation do
 
     type_title = department[0].upcase + department[1..-1]
     filename = "cops_#{department.downcase}.md"
-    content = "#### Department [#{type_title}](#{filename})\n\n".dup
+    content = +"#### Department [#{type_title}](#{filename})\n\n"
     selected_cops.each do |cop|
       anchor = cop.cop_name.sub('/', '').downcase
       content << "* [#{cop.cop_name}](#{filename}##{anchor})\n"
@@ -219,7 +219,7 @@ task generate_cops_documentation: :yard_for_generate_documentation do
   def print_table_of_contents(cops)
     path = "#{Dir.pwd}/manual/cops.md"
     original = File.read(path)
-    content = "<!-- START_COP_LIST -->\n".dup
+    content = +"<!-- START_COP_LIST -->\n"
 
     content << table_contents(cops)