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.
Nice! You learn something new about Ruby every day! I've never used `all?` before. Want me to update with a new pr?