OutputPage: Add addHeadItems() method

OutputPage: Add addHeadItems() method

Right now OutputPage requires head items to have a name when using
the addHeadItem() method.

This in contrary to most of the helper methods, which don't require
a name. ParserOutput::addHeadItem() also doesn't require a name.

Since the existing signature isn't easily changed without adding
confusion, add a separate method that allow numerical indexes
to be pushed.

This ability was already exposed via addParserOutputMetadata()
and its array_merge. This method exposes it directly.

Change-Id: I433afd124b57526caa13a540cda48ba2b99a9bde