Truncate margins at unforced page/column breaks

[kwkbtr]

Spec: CSS Fragmentation Module Level 3 - Adjoining Margins at Breaks

When an unforced break occurs between block-level boxes, any margins adjoining the break truncate to the remaining fragmentainer extent before the break, and are truncated to zero after the break. When a forced break occurs there, adjoining margins before the break are truncated, but margins after the break are preserved. Cloned margins are always truncated to zero on block-level margins.

[kwkbtr]

Fixed by #186.