Added strictSections configuration.

Section behavior used to be somewhat hackily determined from nullValue and
defaultValue, but it was messy. Now it has its own configuration and things are
more explicit.

Previously missing/null sections were treated as false, but if you set
nullValue to something, then missing sections became an error, but null
sections remained false. If you set defaultValue to something, missing sections
went back to being omitted. Kooky.

Now missing sections are omitted, unless you set strictSections to true, in
which case they become an error. nullValue and defaultValue have no impact on
section behavior.

There is no way to make null sections an error, but I don't see any demand for
that, so I'm going to leave that as a TODO.

Closes #60.