New exercise for naming n-sided polygons

Adds a basic exercise for naming polygons 3-8 sides as per https://trello.com/c/gXAZolTw

I'm struggling to make the hints any more helpful than they are now (which is basic)—I'm not sure if going from number to greek to final word is a useful process; they just need to know them.

I figured that spelling would be the greatest challenge in here for most students so I built in a quick Levenshtein distance algorithm. If a student enters a guess that is within three changes of a valid answer it will prompt them to check their spelling rather than firing it back incorrect.

I'm not sure where this goes in the curriculum (or even if there is a place), but it was a relatively easy request to fill.