Add additional methods to Board prototype

Includes methods to:
* return row, col, or block for specific cellId
* return an array containing cell id's of 'neighbors', i.e. cells that
  share a row, col, or block with passed-in cell.
* return a 'conflict' object (with row, col, and block boolean
  properties) if any neighbors already have the proposed cell
  value.

These should be useful for figuring out which cells to highlight
relative to the currently selected cell, and identify and style
any neighbors that are in conflict with the chosen value.