Add Rubocop and sensible default rules.

.rubocop.yml

@@ -0,0 +1,27 @@
+AllCops:
+  TargetRubyVersion: 2.4
+  Exclude:
+    - '**/bin/**/*'
+    - '**/db/**/*'
+    - '**/script/setup'
+    - '**/vendor/**/*'
+
+Lint/AssignmentInCondition:
+  Enabled: false
+
+Metrics/BlockLength:
+  Enabled: false
+
+Metrics/LineLength:
+  Max: 100
+  IgnoredPatterns: ['\A\s*#']
+
+Style/FrozenStringLiteralComment:
+  Enabled: false
+
+Style/StringLiterals:
+  EnforcedStyle: double_quotes
+
+Style/TrailingCommaInLiteral:
+  EnforcedStyleForMultiline: consistent_comma
+

Gemfile

@@ -3,5 +3,6 @@ source "https://rubygems.org"
 group :test do
   gem "minitest", "~> 5.10.3"
   gem "rake"
+  gem "rubocop", "~> 0.50.0"
   gem "safe_yaml", "~> 1.0.4"
 end

Gemfile.lock

@@ -1,16 +1,33 @@
 GEM
   remote: https://rubygems.org/
   specs:
+    ast (2.3.0)
     minitest (5.10.3)
+    parallel (1.12.0)
+    parser (2.4.0.0)
+      ast (~> 2.2)
+    powerpack (0.1.1)
+    rainbow (2.2.2)
+      rake
     rake (12.0.0)
+    rubocop (0.50.0)
+      parallel (~> 1.10)
+      parser (>= 2.3.3.1, < 3.0)
+      powerpack (~> 0.1)
+      rainbow (>= 2.2.2, < 3.0)
+      ruby-progressbar (~> 1.7)
+      unicode-display_width (~> 1.0, >= 1.0.1)
+    ruby-progressbar (1.9.0)
     safe_yaml (1.0.4)
+    unicode-display_width (1.3.0)
 
 PLATFORMS
   ruby
 
 DEPENDENCIES
   minitest (~> 5.10.3)
   rake
+  rubocop (~> 0.50.0)
   safe_yaml (~> 1.0.4)
 
 BUNDLED WITH