Exclude a subset of nodes selected by NodeSelector

[g-pavlov]

It is convenient to grab a document structure that is good enough as it stands and pull it with a node selector, but sometimes only a subset may be necessary. It should be possible to exclude document nodes from a structure built by a node selector specifying the pattern in the document structure manifest's node selector element.

[g-pavlov]

depth and excludePaths are available now to filter nodes.
Exclusion by frontmatter is under discussion and will be tracked separately.

Fixed by: #54