Setting a template based on YAML metadata blocks

[JJ <jj-pl4xscvadcE@public.gmane.org>]

Hello, I've been acquainting myself with pandoc for the past few days.
It's quite neat. I particularly do like pandoc's markdown flavour:
it's everything I've ever wanted out of such an extension! I had some
questions that I've been unable to find answers to in the manpage or
elsewhere, however.

Is it possible to specify a template that pandoc is to use in a YAML
metadata block? Presumably this would only work with --standalone.
I've attempted `template: ` after seeing it elsewhere and hoping it
may work, but no dice. This seems like a common enough use case that
I'd imagine there is an option I am missing: or perhaps people on this
mailing list have external solutions. (though I was hoping to avoid
the use of sed: for no reason other than I've only had to use pandoc
in my script so far, which has been quite neat)

Also, what is best practice for "including" HTML fragments in pandoc?
I've found nothing in pandoc for this purpose so far - which, well,
makes a lot of sense. I was planning on first trying to rework things
to rely entirely on templates, and then in the event of failure just
sed-ing it up. But I harbour some concerns about accidentally
replacing text strings or code blocks.

Also also I am curious: what is the background behind *default* html
templates needing the .html5 extension, but regular html templates
using .html? This threw me for a bit of a loop for a bit.

-- JJ