Add finally to promise-like object.

Closes #680

mithril.js

@@ -1055,6 +1055,14 @@ var m = (function app(window, undefined) {
 			return propify(promise.then(resolve, reject), initialValue);
 		};
 		prop["catch"] = prop.then.bind(null, null);
+		prop["finally"] = function(callback){
+			var _callback = function(value){return m.deferred().resolve(callback(value)).promise;};
+			return prop.then(function(value) {
+				return propify(_callback(value).then(function() {return value;}), initialValue);
+			}, function(reason) {
+				return propify(_callback(reason).then(function() {throw new Error(reason);}), initialValue);
+			});
+		};
 		return prop;
 	}
 	//Promiz.mithril.js | Zolmeister | MIT