epic: enable the creation of IBCS tables

[machow]

The International Business Communication Standards (IBCS) Association has a number of templates for reporting business financials. A user on discord asked whether we can produce these.

I suspect we're fairly close, but would need to complete a few pieces, like:

• Summaries: a way to include summaries (related to #172. But we could also allow people to bring their own summaries (either via styles, or some summary frame API, where people bring their own aggregations).
• Plot: Waterfall sparkplot (seems very doable; but we should just roll this into its own library as a POC)
• Group labels: what part of the table structure is "Gross Profit" (a label for the summary row?).
  • This means that rows are grouped, the but the group name is not shown
  • Could be that when adding a summary, user can indicate "put the group name in the summary row"
• Vertical gutters between columns (could be made by styling vertical borders wider and transparent)
• Bottom borders: Thicker lines for the 2014, 2015 columns on the bottom (is this table_border_bottom in tab_styles()?)