Allow forcing column layout with `!col <name>` in input

[tablify] / Changelog

## [Unreleased]
- Read column threshold from `!col_threshold <N>` in input
- Allow forcing column layout with `!col <name>` in input

## [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