Robustness: Prevent duplicate timer loops and detached DOM memory leaks

[mozluk]

Why

Every invocation of the plugin previously created a new interval that retained the selected DOM collection indefinitely, leading to memory leaks and duplicate timer execution.

How
Stored a single timer instance per element using jQuery's .data("timeago-timer"). Added logic to explicitly clear any existing timer (clearInterval(existingTimer)) before initializing a replacement. Introduced an attachment check (isAttached) inside the refresh loop that automatically stops the interval and cleans up the timer data once the element is detached from the document.

Security/Robustness changes
Eliminates client-side memory leaks and CPU exhaustion by ensuring that detached DOM nodes can be properly garbage collected.

Testing
Verified that jquery-timeago/jquery.timeago.js passes standard node --check validation.