remove the <p> for lone images

Rules

@@ -59,6 +59,7 @@ compile '/html/*' do
         filter :erb # I'll try not to use ruby anymore inside content
         filter :kramdown
         # filter :math_repair # after kramdown
+        filter :fix_img
     else
         raise "Filter is not configured for #{item.identifier} in Rules file."
     end

lib/fix_img.rb

@@ -0,0 +1,8 @@
+class FixImage < Nanoc3::Filter
+    identifier :fix_img
+    def run(content, params={})
+        content.gsub(%r{<p>(<img[^>]*>)</p>}) do |m|
+	        $1.to_s
+		end
+    end
+end