Add column_header_merge_depth and row_height page options

[texodus]

Adds two new non-data-specific configuration options to the schema of the dataListener return object:

• column_header_merge_depth: number configures the number of rows to include
  from colspan merging. This defaults to header_length - 1.
• row_height: number configures the pixel height of a row for
  virtual scrolling calculation. This is typically auto-detected from the DOM,
  but can be overridden if needed.