style: Implement basic column spans, quirks mode, and the legacy `bgcolor` and `border` attributes

This patch provides some of the groundwork for column spans greater than
1. It implements the column-span CSS property as well as the
corresponding colspan attribute; although the former is not
well-specified outside of CSS multi-column layout, INTRINSIC refers to
it. Although width is distributed to spanning columns, they do not yet
contribute minimum and preferred widths; this will be implemented in a
follow-up.

The parsing for the legacy bgcolor and border attributes is
implemented according to the WHATWG HTML specification.

Additionally, this patch cleans up some miscellaneous formatting issues,
refactors layout/css somewhat to eliminate needless levels of
indirection, and cleans up the handling of table rowgroups.

New Hacker News screenshot: http://i.imgur.com/hnl2a7E.png