Support dual-value affiliated keywords

Closes #25

Author: amake

lib/src/org/grammar.dart

@@ -419,8 +419,27 @@ class OrgContentGrammarDefinition extends GrammarDefinition {
       ref0(affiliatedKeywordValue));
 
   Parser affiliatedKeywordKey() =>
+      ref0(affiliatedKeywordDualKey) | ref0(affiliatedKeywordSingleKey);
+
+  Parser affiliatedKeywordSingleKey() =>
       string('#+') & whitespace().neg().starLazy(char(':')) & char(':');
 
+  // See `org-element-dual-keywords`
+  Parser affiliatedKeywordDualKey() =>
+      string('#+') &
+      (string('CAPTION', ignoreCase: true) |
+          string('RESULTS', ignoreCase: true)) &
+      ref0(affiliatedKeywordSecondaryValue).optional() &
+      char(':');
+
+  Parser affiliatedKeywordSecondaryValue() =>
+      char('[') &
+      // TODO(aaron): This end condition handles some edge cases but for full
+      // fidelity to `org-element-affiliated-re` we would need to support
+      // backtracking
+      any().starLazy(string(']:') & (whitespace() | lineEnd())) &
+      char(']');
+
   Parser affiliatedKeywordValue() => any()
       .starLazy(lineEnd())
       .flatten(message: 'Affiliated keyword value expected');

test/org/grammar/affiliated_keyword_test.dart

@@ -38,6 +38,30 @@ void main() {
         ''
       ]);
     });
+    test('dual-value', () {
+      final result = parser.parse('#+caption[blah]: foo');
+      expect(result.value, [
+        '',
+        ['#+caption[blah]:', ' foo'],
+        ''
+      ]);
+    });
+    test('dual-value-capable but single-value', () {
+      final result = parser.parse('#+CAPTION: foo');
+      expect(result.value, [
+        '',
+        ['#+CAPTION:', ' foo'],
+        ''
+      ]);
+    });
+    test('dual-value edge case', () {
+      final result = parser.parse('#+caption[:blah]: ]: foo');
+      expect(result.value, [
+        '',
+        ['#+caption[:blah]: ]:', ' foo'],
+        ''
+      ]);
+    }, skip: "I don't think we can support this without backtracking");
     test('not at beginning of line', () {
       final result = parser.parse('''a   #+blah''');
       expect(result, isA<Failure>());