Add a ReferenceFilter base class

This will allow for the removal of a lot of duplication from the
reference filters.

lib/gitlab/markdown/reference_filter.rb

+require 'html/pipeline'
+
+module Gitlab
+  module Markdown
+    # Base class for GitLab Flavored Markdown reference filters.
+    #
+    # References within <pre>, <code>, <a>, and <style> elements are ignored.
+    #
+    # Context options:
+    #   :project (required) - Current project, ignored when reference is
+    #                         cross-project.
+    #   :reference_class    - Custom CSS class added to reference links.
+    #   :only_path          - Generate path-only links.
+    #
+    class ReferenceFilter < HTML::Pipeline::Filter
+      # Don't look for references in text nodes that are children of these
+      # elements.
+      IGNORE_PARENTS = %w(pre code a style).to_set
+
+      def ignored_ancestry?(node)
+        has_ancestor?(node, IGNORE_PARENTS)
+      end
+
+      def project
+        context[:project]
+      end
+
+      def reference_class(type)
+        "gfm gfm-#{type} #{context[:reference_class]}".strip
+      end
+
+      # Iterate through the document's text nodes, yielding the current node's
+      # content if:
+      #
+      # * The `project` context value is present AND
+      # * The node's content matches `pattern` AND
+      # * The node is not an ancestor of an ignored node type
+      #
+      # pattern - Regex pattern against which to match the node's content
+      #
+      # Yields the current node's String contents. The result of the block will
+      # replace the node's existing content and update the current document.
+      #
+      # Returns the updated Nokogiri::Document object.
+      def replace_text_nodes_matching(pattern)
+        doc.search('text()').each do |node|
+          content = node.to_html
+
+          next if project.nil?
+          next unless content.match(pattern)
+          next if ignored_ancestry?(node)
+
+          html = yield content
+
+          next if html == content
+
+          node.replace(html)
+        end
+
+        doc
+      end
+
+      def validate
+        needs :project
+      end
+    end
+  end
+end