Update Style class

lib/models/documents/attribute.dart

@@ -41,6 +41,8 @@ class Attribute<T> {
 
   static BlockQuoteAttribute blockQuoteAttribute = BlockQuoteAttribute();
 
+  bool get isInline => scope == AttributeScope.INLINE;
+
   static Attribute fromKeyValue(String key, dynamic value) {
     if (!_registry.containsKey(key)) {
       throw ArgumentError.value(key, 'key "$key" not found.');

lib/models/documents/style.dart

@@ -19,4 +19,18 @@ class Style {
     });
     return Style.attr(result);
   }
+
+  Iterable<String> get keys => _attributes.keys;
+
+  Iterable<Attribute> get values => _attributes.values;
+
+  bool get isEmpty => _attributes.isEmpty;
+
+  bool get isNotEmpty => _attributes.isNotEmpty;
+
+  bool get isInline => isNotEmpty && values.every((item) => item.isInline);
+
+  Attribute get single => _attributes.values.single;
+
+  bool containsKey(String key) => _attributes.containsKey(key);
 }