Don't color the background of cells that contain only '×'s

[tablify] / Changelog

## [Unreleased]
- `!label col:new label` to substitute a different label for a column
- Use Unicode tally marks to coalesce multiple no-colon entries
  (If you need a font for these, I recommend "BabelStone Han")
- Don't color the background of cells that contain only '×'s

## [0.4.0] - 2024-10-03
- Read column threshold from `!col_threshold <N>` in input
- Allow forcing column layout with `!col <name>` in input
- `!colsep` to separate `!col` columns
- `!hide` to hide a column

## [0.3.0] - 2024-10-02
- Center text in each cell
- Escape HTML characters properly
- Fix an unnecessary O(n^2)ism
- Rare events appear as end-of-line notes rather than mostly-empty columns
- Double blank line for small break between rows

## [0.2.1] - 2024-08-20
- A little more space up top
- Sort columns _descending_ by frequency

## [0.2.0] - 2024-08-19
Initial release