Feature pages: Sort spells by level and name

[PJBrs]

These four patches help optionally sorting spells by both level and name, instead of by name only. I'll admit that the first patch is slightly off-topic.

The first patch adds a short overview of a character's spellcasting details to the spellcasting section of the feature pages, see example:

The second patch adds an commandline option to sort spells by level and name: --spells-by-level, -S Order spells by level in the feature pages and implements it.

The third patch implements headers and pdf bookmarks for each spell level in the feature pages.

The fourth patch adds documentation.

These patches work for both the regular and the fancy latex pages.