HTMLWriter uses the document to look up export options and adapt it's
behaviour. The writer.document & log are set in the Before() method during
normal use via document.Write().
Sometimes the writer is used separately and won't have it's Before method
called - in those cases we should use the defaults rather than crashing.
TIL embedded types can be acessed by their non-namespaced name - so there
actually is no need for a type alias to access .strings.Builder, one can just
use .Builder.
The existing approach made it hard to extend existing writers.
With this change, replacing individual methods of a writer is possible by
embedding it.
Sharing the WriteNodes function also removes some unnecesseray duplication, so
win win.
Hugo has some hardcoded checks that have to be fulfilled in its Table of
Contents extraction workflow (helpers/content ExtractTOC). It's easier this
way...
