Allow users to pass an index_path or array of index_paths to update_table_data.

When this is passed, it won't update the entire table data, but just the cell or cells that correspond to the index_path(s) passed.

Don't merge now. Just opening for discussion. Needs tests too.
This method should probably go into an array class extension, but we don't do that, so i put it here... let me know if there's an easier way to do this with ruby in a one liner that i don't know about.