Add markdownlinter gem to development group

Gemfile

@@ -104,6 +104,7 @@ group :test do
 end
 
 group :development do
+  gem 'mdl', require: false
   # Access an IRB console on exception pages or by using <%= console %> in views
   gem 'web-console', '3.3.0'
 end

Gemfile.lock

@@ -224,6 +224,7 @@ GEM
     knapsack (1.13.3)
       rake
       timecop (>= 0.1.0)
+    kramdown (1.13.2)
     launchy (2.4.3)
       addressable (~> 2.3)
     letter_opener (1.4.1)
@@ -236,12 +237,18 @@ GEM
       nokogiri (>= 1.5.9)
     mail (2.6.6)
       mime-types (>= 1.16, < 4)
+    mdl (0.4.0)
+      kramdown (~> 1.12, >= 1.12.0)
+      mixlib-cli (~> 1.7, >= 1.7.0)
+      mixlib-config (~> 2.2, >= 2.2.1)
     mime-types (3.1)
       mime-types-data (~> 3.2015)
     mime-types-data (3.2016.0521)
     mimemagic (0.3.2)
     mini_portile2 (2.1.0)
     minitest (5.10.1)
+    mixlib-cli (1.7.0)
+    mixlib-config (2.2.4)
     multi_json (1.12.1)
     multi_xml (0.6.0)
     multipart-post (2.0.0)
@@ -514,6 +521,7 @@ DEPENDENCIES
   knapsack
   launchy
   letter_opener_web (~> 1.3.1)
+  mdl
   newrelic_rpm (~> 4.1.0.333)
   omniauth
   omniauth-facebook (~> 4.0.0)