add |= operator

src/BXParser.cc

 		skip(); // '='
 		skip(); // '+'
 		return Operator::EQUALS_PLUS;
+	} else if (peek() == '|' && ch2 == '|') {
+		skip(); // '|'
+		skip(); // '='
+		return Operator::BAR_EQUALS;
 	}
 
 	return Operator::NONE;
 				break;
 
 			default:
-				if (ch == ':' && peek(2) == '=')
+				int ch2 = peek(2);
+				if (
+						(ch == ':' && ch2 == '=') ||
+						(ch == '+' && ch2 == '=') ||
+						(ch == '=' && ch2 == '+') ||
+						(ch == '|' && ch2 == '=')) {
 					return success;
+				}
 
 				content.push_back(get());
 				success = true;
 			values.clear();
 			break;
 
+		case Operator::BAR_EQUALS:
+			{
+				auto &vec = vars[key];
+				for (size_t i = 0; i < vec.size(); ++i) {
+					bool exists = false;
+					for (auto &val: values) {
+						if (val == vec[i]) {
+							exists = true;
+							break;
+						}
+					}
+
+					if (!exists) {
+						values.push_back(std::move(vec[i]));
+					}
+				}
+				vec = std::move(values);
+			}
+			values.clear();
+			break;
+
 		case Operator::NONE:
 			break;
 		}

src/BXParser.h

 		COLON_EQUALS,
 		PLUS_EQUALS,
 		EQUALS_PLUS,
+		BAR_EQUALS,
 		NONE,
 	};