This change allows bibtex/biblatex output to wrap as other
formats do, depending on the settings of `--wrap` and `--columns`.
It also introduces default templates for bibtex and biblatex,
which allow for using the variables `header-include`, `include-before`
or `include-after` (or alternatively the command line options
`--include-in-header`, `--include-before-body`, `--include-after-body`)
to insert content into the generated bibtex/biblatex.
This change requires a change in the return type of the unexported
`T.P.Citeproc.writeBibTeXString` from `Text` to `Doc Text`.
Closes#7068.
