JavaScript redirect to split on missing ID

When you have a document like:

animal.bigb

= Animal

== Dog

=== Poodle

the version without -S, --split-headers will contains a valid ID within it:

animal.html#poodle

However, if at some point you decide that the section dog has become too large and want to split it as:

= Animal

\Include[dog]

and:

dog.bigb

= Dog

== Poodle

When you do this, it would break liks that users might have shared to animal.html#poodle, which is not located at dog.html#poodle.

To make that less worse, if -S, --split-headers are enabled, we check at runtime if the ID poodle is present in the output, and if it is not, we redirect to the split page #poodle to poodle.html.

It would be even more awesome if we were able to redirect to the non-split version as well, dog.html#poodle, but that would be harder to implement, so not doing it for now.