Add support for columnar layout in sections.

API Design

• section.columns — SectionColumns object
• section.columns.count — int (1 = single column, 2+ = multi-column)
• section.columns.space — Emu (space between columns)
• section.columns.equal_width — bool
• section.columns[n] — individual column width/space override when not equal width

XML

w:sectPr > w:cols with attributes: w:num (count), w:space (default spacing), w:equalWidth
Child w:col elements for individual column widths when not equal.

Upstream: python-openxml#167 (12 comments)