v2.19.0

Conditional formatting rules now survive a round trip intact.

Fixed

• Six of the eleven rule kinds were written back incomplete. The reader
  modelled top10, aboveAverage, containsText, notContainsText,
  beginsWith, endsWith, timePeriod, duplicateValues and uniqueValues,
  but the writer emitted only the type, style and priority for them: no rank,
  no text, no stdDev, no timePeriod, and no formula children. Opening a
  workbook that used one of those rules and saving it back produced a cfRule
  Excel could not act on. A below-average rule was the worst case, because
  losing aboveAverage="0" silently inverted it.
• The reader now captures text, rank, percent, bottom, aboveAverage,
  equalAverage, stdDev and timePeriod, the writer emits them, and rules
  that carry a formula alongside their attributes keep it.

New

• Authoring factories for the rule kinds that could previously only be read:
  ConditionalFormat.containsText, .notContainsText, .beginsWith,
  .endsWith, .top10 (with percent and bottom), .aboveAverage (with
  below, orEqual and standardDeviations), .duplicateValues,
  .uniqueValues and .timePeriod.
• The matching detail is exposed on ConditionalFormat: text, rank,
  rankIsPercent, rankFromBottom, aboveAverage, equalAverage, stdDev
  and timePeriod.

Excel stores a text rule as both an attribute and an equivalent formula and
expects the two to agree, so the factories generate the formula for you.