Add hero_button to template YAML options

R/bulma_document.R

 #' Bulma Document Renderer
 #'
-#' TODO Document YAML options here.
+#' @section YAML options:
+#' You can pass additional options to the underlying template via the `bulma`
+#' YAML item. The following options can be specified there.
+#' \itemize{
+#'   \item{`hero`: Classes applied to the hero containing the title}
+#'   \item{`hero_button`: Classes applied to the `header_links` buttons}
+#' }
 #'
 #' @inheritDotParams rmarkdown::html_document_base
 #' @export

inst/bulma/bulma.html

 $endif$
 $if(header-links)$
 $for(header-links)$
-<a class="button is-primary is-inverted is-outlined" href="$header-links.url$">$header-links.name$</a>
+<a class="button $if(bulma.hero_button)$$for(bulma.hero_button)$$bulma.hero_button$$sep$ $endfor$$else$is-primary is-inverted is-outlined$endif$" href="$header-links.url$">$header-links.name$</a>
 $endfor$
 $endif$ <!-- header-links -->
 </div>

tests/manual/layout/level/bulma_layout_level.Rmd

     extra: 
       - "Second Affiliation"
       - "Very Cool Guy"
-date: '`r strftime(Sys.time(), "%F %T")`'
+date: '`r strftime(Sys.time(), "%A, %B %e, %Y")`'
 header-links:
   - name: Github
     url: https://github.com/gerkelab
   - name: Home
     url: https://gerkelab.com
 bulma:
-  hero: ["info", "bold"]
+  hero: ["dark"]
+  hero_button: ["is-secondary"]
 output: bulma::bulma_document
 editor_options: 
   chunk_output_type: console