Do bibliography commands logic in R instead of doing in TeX

The convoluted logic using \ifboolexpr and \ifstrequal in the preamble of
template.tex is done now done in function tf_article(). A new pandoc
variable ("biblio-commands") is now used.

version 0.27

Update to version 0.26

Bump version

Release 0.24

release rticles 0.23

Bump version

CRAN release v0.21

CRAN release v0.20

CRAN release v0.19