Added headers align, upgraded column align.

changes in:
    _normalize_tabular_data:
        - also returns headers_pad (number of first columns without header)
    tabulate: 
        - new keyword-arguments: `colglobalalign`, `headersglobalalign` and `headersalign`
        - add "global" support for colalign
        - updated tabulate.__doc__
    _format_table:
        - takes argument 'headersaligns'

Author: eliegoudout