Add preprocessor to make cells undeletable

This closes #6. It's a pretty simple preprocessor at this point, but I'm thinking in the future we might want to extend it's functionality to do thinks like compute a checksum of the cell contents, to verify later during grading. This is also why I chose the name `LockCells` as opposed to `MarkCellsUndeletable` or something, because in the future its functionality will probably be reflected more accurately by the term "locked".