Filter in insert_chat_line. Fixes #11.

As suggested in #11, we can get rid of the timer-based filtering and use a "monkey patch" instead. That is, we can replace the original `CurrentChat.insert_chat_line` with our own function. Inside our own function, we call the original version plus some extra code to add the filter classes.

Since we're using a monkey patch, we have access to the *unformatted* chat message through the `e` argument of our `insert_chat_line` function. We don't even need any `$.text()` calls, we can just use `e.message`.

However, we can't simply go and look for the last `li` item in `#chat_line_list` to add our filter classes. The item is not even inserted in the DOM yet! Here's how I got around that:

The original `insert_chat_line` calls `insert_with_lock`. This adds an "operation" object to `CurrentChat.queue` describing the item to be inserted and sets a timer to actually perform those insert operation (if such a timer is not yet set).

After `insert_chat_line` is called, we know that an new chat line is added to the queue. We can thus retrieve that item and *modify* it! The `line` property holds the HTML to be inserted, thus a simple `String.replace` is sufficient to modify the class.