Added 'custom' attribute for non-UI attributes

.gitignore

@@ -1,2 +1,4 @@
+.idea/
+packages/zefyr/example/ios/Flutter/flutter_export_environment.sh
 .DS_Store
 .idea/

doc/concepts/attributes.md

@@ -19,7 +19,8 @@ attribute can be either *inline-scoped* or *line-scoped*, but not both.
 A good example of an inline-scoped attribute is "bold" attribute. Bold
 style can be applied to any character within a line, but not to the
 line itself. Similarly "heading" style is line-scoped and has effect
-only on the line as a whole.
+only on the line as a whole.  A custom *inline-scoped* attributes can be
+added for tracking of non-UI based attributes.
 
 Below table summarizes information about all currently supported
 attributes in Zefyr:
@@ -32,6 +33,7 @@ attributes in Zefyr:
 | Heading | `heading` | `line`   | `int`    | `1`, `2` and `3`                       |
 | Block   | `block`   | `line`   | `String` | `"ul"`, `"ol"`, `"code"` and `"quote"` |
 | Embed   | `embed`   | `inline` | `Map`    | `"hr"`, `"image"`                      |
+| Custom  | `custom`  | `inline` | `String` | Non-empty string                       |
 
 Removing a specific style is as simple as setting corresponding
 attribute to `null`.
@@ -68,6 +70,9 @@ void makeItPretty(NotusDocument document) {
   // Remove heading style from the first line. All attributes
   // have `unset` property which can be used the same way.
   document.format(0, 0, NotusAttribute.heading.unset);
+
+  // Add a custom attribute that does not reflect in the UI
+  document.format(15, 23, NotusAttribute.custom.withValue('customAttribute'));
 }
 ```
 

packages/notus/example/main.dart

@@ -11,6 +11,7 @@ void main() {
       0, 'Notus package provides rich text document model for Zefyr editor');
   doc.format(0, 5, NotusAttribute.bold); // Makes first word bold.
   doc.format(0, 0, NotusAttribute.h1); // Makes first line a heading.
+  doc.format(15, 23, NotusAttribute.custom.withValue('customAttribute')); // Adds a non-UI custom attribute
   doc.delete(23, 10); // Deletes "rich text " segment.
 
   // Collects style attributes at 1 character in this document.

packages/notus/lib/src/document/attributes.dart

@@ -77,6 +77,7 @@ class NotusAttribute<T> implements NotusAttributeBuilder<T> {
     NotusAttribute.heading.key: NotusAttribute.heading,
     NotusAttribute.block.key: NotusAttribute.block,
     NotusAttribute.embed.key: NotusAttribute.embed,
+    NotusAttribute.custom.key: NotusAttribute.custom,
   };
 
   // Inline attributes
@@ -126,6 +127,9 @@ class NotusAttribute<T> implements NotusAttributeBuilder<T> {
   // ignore: const_eval_throws_exception
   static const embed = EmbedAttributeBuilder._();
 
+  /// Custom attribute.
+  static final custom =  CustomAttributeBuilder._();
+
   static NotusAttribute _fromKeyValue(String key, dynamic value) {
     if (!_registry.containsKey(key)) {
       throw ArgumentError.value(
@@ -454,3 +458,16 @@ class EmbedAttribute extends NotusAttribute<Map<String, dynamic>> {
     return hashObjects(objects);
   }
 }
+
+/// Builder for custom attributes that do not necessarily need styling.
+/// 
+/// There is no need to use this class directly, consider using
+/// [NotusAttribute.custom] instead.
+class CustomAttributeBuilder extends NotusAttributeBuilder<String> {
+  static const _kCustom = 'custom';
+
+  const CustomAttributeBuilder._() : super._(_kCustom, NotusAttributeScope.inline);
+
+  NotusAttribute<String> withValue(String value) => NotusAttribute<String>._(key, scope, value);  
+}
+