Add YamlFrontMatterFilter to the PreProcessPipeline

This filter will detect YAML Front Matter and convert it to an HTML
table for prettier formatting.

lib/banzai/filter/yaml_front_matter_filter.rb

+require 'html/pipeline/filter'
+require 'yaml'
+
+module Banzai
+  module Filter
+    class YamlFrontMatterFilter < HTML::Pipeline::Filter
+      DELIM = '---'.freeze
+
+      # Hat-tip to Middleman: https://git.io/v2e0z
+      PATTERN = %r{
+        \A(?:[^\r\n]*coding:[^\r\n]*\r?\n)?
+        (?<start>#{DELIM})[ ]*\r?\n
+        (?<frontmatter>.*?)[ ]*\r?\n?
+        ^(?<stop>#{DELIM})[ ]*\r?\n?
+        \r?\n?
+        (?<content>.*)
+      }mx.freeze
+
+      def call
+        match = PATTERN.match(html)
+
+        return html unless match
+
+        "```yaml\n#{match['frontmatter']}\n```\n\n#{match['content']}"
+      end
+    end
+  end
+end

lib/banzai/pipeline/pre_process_pipeline.rb

@@ -2,7 +2,8 @@ module Banzai
   module Pipeline
     class PreProcessPipeline < BasePipeline
       def self.filters
-        [
+        FilterArray[
+          Filter::YamlFrontMatterFilter
         ]
       end
 

spec/lib/banzai/filter/yaml_front_matter_filter_spec.rb

+require 'rails_helper'
+
+describe Banzai::Filter::YamlFrontMatterFilter, lib: true do
+  include FilterSpecHelper
+
+  it 'allows for `encoding:` before the frontmatter' do
+    content = <<-MD.strip_heredoc
+      # encoding: UTF-8
+      ---
+      foo: foo
+      ---
+
+      # Header
+
+      Content
+    MD
+
+    output = filter(content)
+
+    expect(output).not_to match 'encoding'
+  end
+
+  it 'converts YAML frontmatter to a fenced code block' do
+    content = <<-MD.strip_heredoc
+      ---
+      bar: :bar_symbol
+      ---
+
+      # Header
+
+      Content
+    MD
+
+    output = filter(content)
+
+    aggregate_failures do
+      expect(output).not_to include '---'
+      expect(output).to include "```yaml\nbar: :bar_symbol\n```"
+    end
+  end
+
+  context 'on content without frontmatter' do
+    it 'returns the content unmodified' do
+      content = <<-MD.strip_heredoc
+        # This is some Markdown
+
+        It has no YAML frontmatter to parse.
+      MD
+
+      expect(filter(content)).to eq content
+    end
+  end
+end