Instead of hard-coding the formatting in the block, we
now use a show rule on `<refs>`, included conditionally by
the default template.
Values will be set for `csl-hanging-indent` and
`csl-entry-spacing` based on the CSL style, but these
variables can be overridden on the command line using
`--variable`.
It is also possible to include a new show rule in
header-includes, which will take priority over the other.
So users have maximum flexibility in styling bibliographies.
Closes#11926.
John MacFarlane
9d4a69fe01