Removed unnecessary CSS rule

The invalid class is not being used.
Give "cell" class to cells

This is for readability, and maybe avoiding ambiguity if table cells
are ever used for anything else.
Added labels to cells

This uses the notation I have sometimes found: rows are given a letter
from “a” to “i”, columns are given a number from 1 to 9, and cells are
given a letter and a number.
Tweaked populateGrid and moveIsValid methods

I am using the cell labels instead of the row and column indexes.
Keypress changes
Tweaked code used to get rows, columns, and boxes

It is a little bit more complex, but hopefully it should be less
confusing since I am using the row and column indexes I am switching
to, instead of the 0-based indexes I was using before.
Added property to plugin for cells

This may slightly speed up and simplify things.
Added method to get a cell

For readability.
Renamed some variables

For consistency.
Created some useful constants

Also started using them. They may not have the best names or be in the
best place but they are good enough for my purposes right now.
Simplified moveIsValid method
Renamed some constants

It should now be clear which ones are plain arrays and which ones are
associative arrays.
Tweaked findConflicts method

It uses my new constants. The rows, columns, and boxes properties of my
plugin may no longer be needed.
Added getCells method

This method gets all cells whose cell labels match any cell label in an
array passed to it.
Removed rows, columns, and boxes properties
Added getInitialDigitHash method

It returns an object whose keys are the cell labels of the given cells
and whose values are the digits in those cells.
Exposed plugin as a global object

This is only for testing. I may remove it later.
Tweaked handling of empty arrays

I think it is more readable.
Several tweaks to code that generates constants
New capability for populateGrid

It can now populate the grid based on a hash with cell labels and
digits. If no hash is passed, it will populate at random like before.