Expand justification.

[Robpol86]

Currently we only have:
self.justify_columns = dict() # {0: 'right', 1: 'left', 2: 'center'}

Add:
self.justify_rows = dict() # {0: 'top', 1: 'middle', 2: 'bottom'}
self.justify_cells = dict() # {(0, 0): ('center', 'middle')}

justify_columns and justify_rows override defaults (left and top is the default). justify_cells overrides defaults for specific cells. Keys are (x, y) coords from left/top corner and values are one or two justification values (horizontal or vertical justifications).