Prevent arrow keys from scrolling

Closes #66
Explain CHUNk as soon as we start using it.

Closes #66
Merge pull request #89 from zspencer/66-describe-chunk-in-lesson-2

Explain CHUNK as soon as we start using it.
Merge pull request #87 from zspencer/stop-arrow-keys-from-moving-browser-window

Prevent arrow keys from scrolling