feat: get single attribute by key

Sources/AndroidXML/Xml_Parser.swift

@@ -90,6 +90,22 @@ public struct Xml_Parser {
 			// No `lookupFn` because attribute values are likely unique
 			try asDict().mapValues { $0.resolve(stringPool)}
 		}
+		
+		/// Convenience method If you just need a single attribute.
+		/// This is slightly more efficient than generating `asDict()` and then throwing away everything else.
+		/// But if you need two or more values, you should use `asDict()`.
+		public func get(_ attrName: String) throws -> ResValue? {
+			for attr in try element.attributes() {
+				var key = lookupFn(attr.name)
+				if let prefix = namespaces[attr.ns] {
+					key = prefix + ":" + key
+				}
+				if key == attrName {
+					return attr.typedValue
+				}
+			}
+			return nil
+		}
 	}
 	
 	/// Iterate over whole tree and call `start` and `end` blocks for each element.