Support additional Confluence table features

[willkg]

GFM tables currently render to plain Confluence storage tables. Confluence supports richer table features that markdown doesn't express natively:

• table/page width (e.g. full-width vs. default)
• cell/column alignment
• cell background color
• explicit column widths

Investigate how to author these in markdown (frontmatter, per-table directives, raw storage passthrough, or an attribute syntax) and how they map to the storage-format table markup (<table>, <colgroup>/<col>, data-highlight-colour, alignment classes, etc.). Deliverable: a design for the authoring surface, then implementation.

[willkg]

PR #50 made the default better so we don't need to implement table width and column widths.

Column alignment would get fixed by issue #48.

I still want to add support for cell background color.