Improved development warnings, better tag method response

Improved Tag response

• PR: #59
• Issue: canjs/can-view-import#2

When attempting to retrieve the handle for a given tag that has not been registered, like tag('hello-world'), the tag method would return a noop. In some cases, this made it difficult to determine if a tag exists or not since a tag that exists returns a function as well.

Now, can-view-callbacks exports a default noop function, defaultCallback so a comparison can be made to the result of the tag method.

import callbacks from 'can-view-callbacks';
const tagHandler = tag('hello-world');
const tagNotRegistered = callbacks.defaultCallback === tagHandler;

Improved development warnings

• PR: #58
• Issue: #57

A custom element attribute must be registered before it is rendered on an element. Previously, rendering an element with a custom attribute would not log a warning. A warning was added with a reminder that the attribute must be defined before it is used.