- Added event listeners for mouseover and mouseout of tokens to
  implement highlighting feature. The event listeners trigger
  the highlightTokens and unHighlightTokens functions which change
  the background-color style property of the moused over token and the
  ones above it.
- Added highlighting of computer's selection
- Added token array indices as parameters to the highlight functions
  so that they may be called using the computer's selection
- When computer wins, does not properly detect that all tokens are
  removed, perhaps because tokens.length==0 is checked before the tokens
  are actually removed due to the setTimeout on removeTokens. Does the
  control flow wait 2s, steps inside function, and returns to the spot
  following setTimeout? Or does it queue the delay to execute 2s later,
  and immediately proceeds with executing instructions following it?
- Getting undefined tokens array length error within unHighlightTokens
  function. Happens after player moves mouse over the empty space
  where the token they selected was before.
- Fixed detection of remaining tokens in startCompTurn function
  by joining the removeTokens call and the check for tokens.length
  together in setTimeout
- Added if statement in unHighLightTokens function that verifies
  the column index of the tokens array is defined to prevent function from
  throwing error of invalid array access