New - rows().cells(), columns.cells() API methods.
    - row options for column selector

- Selectors (table, rows and columns) now held in a single file, sharing
  structure. A more unified API is used, with row options also being
  allow for columns, through the use of the second parameter for the
  columns() method, which will effect how a column function can act upon
  rows (for example, the order of the rows when getting data or nodes).

- Dev: tables() is no longer an iterator - using an `iterator()` method
  with options which are suitable for the different types of iteration
  the API needs.
New: columns().data() API method - get the data for the selected columns