Don't overwrite frontmatter parameters not known to incremental-writing plugin

[chaosmonster]

As a user I want to use the css helper classes provided by a theme like minimal. Those css helper classes are defined in the frontmatter of the file (see example). After adding a note to the queue or getting the next repetition the frontmatter is overwritten by the queues frontmatter and deleted the additional frontmatter information.

example of additional frontmatter info that should survive interacting with the queue

---
cssClass:
  - table-max
  - row-alt
---