Add HideNoNs option

[furbrain]

By default nspages provides a warning "namespace doesn't exist". This is
usually what is wanted. Sometimes users will want to hide this warning -
for example with generated files which contain a ToC for a namespace which
may not yet have been created.

This PR adds a new option "-hideNoNs" which implements this

[gturri]

Thanks for the PR.
(I really appreciate that you wrote some non regression tests 👍 )