[ieee] Improve bibliography handling (#530)

inst/rmarkdown/templates/ieee/resources/template.tex

@@ -329,17 +329,15 @@
 %% BEGIN MY ADDITIONS %%
 
 $if(natbib)$
-\usepackage{natbib}
+\usepackage[$natbiboptions$]{natbib}
 \bibliographystyle{$if(biblio-style)$$biblio-style$$else$plainnat$endif$}
 $endif$
 $if(biblatex)$
-\usepackage[backend=bibtex,citestyle=ieee,style=numeric$if(citation_sorting)$,sorting=$citation_sorting$$endif$]{biblatex}
-$if(bibliography)$
+\usepackage[citestyle=ieee,$if(biblio-style)$style=$biblio-style$,$endif$$if(citation_sorting)$sorting=$citation_sorting$$endif$,$for(biblatexoptions)$$biblatexoptions$$sep$,$endfor$]{biblatex}
 $for(bibliography)$
 \addbibresource{$bibliography$}
 $endfor$
 $endif$
-$endif$
 
 \usepackage[unicode=true]{hyperref}
 
@@ -615,16 +613,15 @@
 $body$
 
 $if(natbib)$
-$if(biblio-files)$
+$if(bibliography)$
 $if(biblio-title)$
-$if(book-class)$
+$if(has-chapters)$
 \renewcommand\bibname{$biblio-title$}
 $else$
 \renewcommand\refname{$biblio-title$}
 $endif$
 $endif$
-\bibliography{$biblio-files$}
-
+\bibliography{$for(bibliography)$$bibliography$$sep$,$endfor$}
 $endif$
 $endif$
 $if(biblatex)$