Allows for link clicks to trigger when editor is not first responder

Allows for link clicks to trigger a window.open call to target _blank. Currently, clicking a link simply puts the editor into isFirstResponder, which is not consistent with user expectation.

The approach here is to simply modify the editor's mouseDown event handler to detect whether the event.target is an anchor tag AND whether the editor has isFirstResponder.

A user story here might be "As a user, if I click a link in the editor while NOT editing content, the link should open a new window/tab and display the linked webpage." And "... if I click a link while IN editing mode, place the caret within the link text so that I can modify the link using the toolbar tool, if needed."

Unit test provided.
I know you probably worked really hard on this unit test, it seems like a very tricky proposition to test this behavior, but I'm sorry, I'm not sure that we should keep it as is. For one, if you make the change below that I suggest (of returning `true` from `mouseDown`) it seems that this test will fail or at least it should fail, but there's an issue with it. The `click` event is handled differently than `mouseDown`/`mouseUp`. The root responder actually returns `false` and stops the normal behavior and propagation of the event on a `click`. So the test isn't actually hitting the rich text editor.

The best I can come up with right now would be to trigger a `mouseDown