Add checks

Gemfile

@@ -12,6 +12,7 @@ gem 'pdfkit'
 gem 'sass'
 gem 'sassc'
 gem 'slim'
+gem 'nokogiri'
 
 group :nanoc do
   gem 'guard-nanoc'

Gemfile.lock

@@ -57,6 +57,7 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.10)
     lumberjack (1.2.4)
     method_source (0.9.2)
+    mini_portile2 (2.4.0)
     mini_racer (0.2.9)
       libv8 (>= 6.9.411)
     multi_json (1.14.1)
@@ -84,6 +85,8 @@ GEM
       tty-platform (~> 0.2)
       zeitwerk (~> 2.1)
     nenv (0.3.0)
+    nokogiri (1.10.10)
+      mini_portile2 (~> 2.4.0)
     notiffany (0.1.3)
       nenv (~> 0.1)
       shellany (~> 0.0)
@@ -137,6 +140,7 @@ DEPENDENCIES
   kramdown
   mini_racer
   nanoc
+  nokogiri
   pdfkit
   sass
   sassc

nanoc.yaml

@@ -15,3 +15,9 @@ prune:
 data_sources:
   - type: filesystem
     encoding: utf-8
+
+checking:
+  enabled_checks:
+    - external_links
+    - internal_links
+    - stale