Feature: validate section coverage

[telic]

Each kind of sectioning should fully-partition a document, and if this is not the case a warning should be logged.

Specifically,

• DIVs should either be used only in FRONTMATTER and BACKMATTER, or contain all text content in the document
• ACTs and SCENEs should contain all text content in the document that is not in FRONTMATTER or BACKMATTER
• PAGEs should contain all text content in the document

Of course, each type may simply not be used at all as well.

[ubermichael]

I don't think this is something we can express in a schema - perhaps a CoverageValidator? I'm not set on that name.

[ubermichael]

This needs a new Vladiator.