Scroll to bottom on new message (if already at bottom).

#### What's this PR do?

If the window was scrolled to the bottom when a new message comes in, then it will scroll back to bottom after adding the new message. This allows the user to view the current messages coming in without having to manually scroll down.

#### Where should the reviewer start?

Look at [onMessage](https://github.com/quickleft/tv/pull/5/files#diff-858d9b3c8622623b0da43bd2ed68d89dL14)