Enabling/disabling documentation based on protection attributes

[kiith-sa]

Currently we generate docs for everything with a doc comment regardless of protection attribute.

Allow teh user to disable private/package/protected.

By default, only public and protected members of non-final classes should be documented.

See DocVisitor.attributes, DocVisitor.writeAttributes(), search for protect in visitor.d for reference.

[kiith-sa]

A better way to do this may be to document everything by default (even undocumented symbols), but if JS is enabled, hide the private/package/undocumented symbols by default, with a button to expand them.

We still should have settings for whether private/package/undocumented should have documentation generated at all.