New: New cell based API methods

- New API methods:
  - cells( rowSelector, columnSelector, opts ) - Cell selector
  - cells( ... ).nodes() - Get the nodes for the selected cells
  - cells( ... ).data() - Get the data for the selected cells
  - cell( rowSelector, columnSelector, opts ) - Single cell selector
  - cell( ... ).node() - Get the cell node
  - cell( ... ).data() - Get the cell data

- There was a bug in `_selector_first` whereby it wasn't correctly
  reducing the set - was gatting away with it before, because it was
  reducing to a single element array, which has a toString() method
  which allowed it to appear to work for integers.