This allows API servers to return URL(x) in the cmdline template,
which pixiecore translates to an appropriate ID(x) template call
to get the file proxied via Pixiecore.
The main difficulty in passing a commandline as a plain string is
that you sometimes want to encode IDs into it, to tell your kernel
to fetch something via Pixiecore. Solving that problem with maps is
a bit clunky. Instead, now we use a text/template {{ ID "foo" }} to
mean "make a URL that, when fetched, reads the file "foo" from the
Booter.
