Slightly optimize max_dimensions().
Slightly optimize max_dimensions().

1. Eliminate append to splitlines() list result.
2. Eliminate two calls to max() for widths.
3. Eliminate bug beause len() was used instead of only visible_width().

New fastest time: 10000 loops, best of 3: 72.2 usec per loop
Old fastest time: 10000 loops, best of 3: 73.6 usec per loop

No real difference but I did fix that bug :P

Robpol86/terminaltables