Add srcset parsing

This commit adds a new ImageElementParser class that encompasses
existing custom `img[alt]` handling with the addition of `srcset`
parsing based on the rules I proposed in
microformats/microformats2-parsing#7.

See my comment here:

microformats/microformats2-parsing#7 (comment)

lib/micro_micro/parsers/image_element_parser.rb

@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+module MicroMicro
+  module Parsers
+    class ImageElementParser
+      # @return [String]
+      attr_reader :value
+
+      # @param node [Nokogiri::XML::Element]
+      # @param value [String]
+      def initialize(node, value)
+        @node = node
+        @value = value
+      end
+
+      # @return [String, nil]
+      def alt
+        @alt ||= node['alt']&.strip
+      end
+
+      # @return [Boolean]
+      def alt?
+        !alt.nil?
+      end
+
+      # @return [Hash{Symbol => String}, nil]
+      def srcset
+        @srcset ||= parsed_srcset if node['srcset']
+      end
+
+      # @return [Boolean]
+      def srcset?
+        srcset.present?
+      end
+
+      # @return [Hash{Symbol => String, Hash{Symbol => String}}]
+      def to_h
+        hash = { value: value }
+
+        hash[:srcset] = srcset if srcset?
+        hash[:alt] = alt if alt?
+
+        hash
+      end
+
+      private
+
+      # @return [Nokogiri::XML::Element]
+      attr_reader :node
+
+      # @return [Hash{Symbol => String}]
+      def parsed_srcset
+        node['srcset']
+          .split(',')
+          .to_h do |candidate|
+            candidate.strip.match(/^(.+?)(\s+.+)?$/) do
+              [(Regexp.last_match(2) || '1x').strip.to_sym, Regexp.last_match(1)]
+            end
+          end
+      end
+    end
+  end
+end

lib/micro_micro/parsers/implied_photo_property_parser.rb

@@ -19,12 +19,11 @@ class ImpliedPhotoPropertyParser < BaseImpliedPropertyParser
       def value
         @value ||=
           if attribute_value
-            return attribute_value unless candidate_node.matches?('img[alt]')
-
-            {
-              value: attribute_value,
-              alt: candidate_node['alt'].strip
-            }
+            if candidate_node.matches?('img[alt], img[srcset]')
+              ImageElementParser.new(candidate_node, attribute_value).to_h
+            else
+              attribute_value
+            end
           end
       end
 

lib/micro_micro/parsers/url_property_parser.rb

@@ -23,11 +23,8 @@ class UrlPropertyParser < BasePropertyParser
       # @return [String, Hash{Symbol => String}]
       def value
         @value ||=
-          if node.matches?('img[alt]')
-            {
-              value: resolved_value,
-              alt: node['alt'].strip
-            }
+          if node.matches?('img[alt], img[srcset]')
+            ImageElementParser.new(node, resolved_value).to_h
           else
             resolved_value
           end

lib/micromicro.rb

@@ -15,6 +15,7 @@
 require_relative 'micro_micro/helpers'
 
 require_relative 'micro_micro/parsers/date_time_parser'
+require_relative 'micro_micro/parsers/image_element_parser'
 require_relative 'micro_micro/parsers/value_class_pattern_parser'
 
 require_relative 'micro_micro/parsers/base_property_parser'