Fix #6 remove automatic dependency on Font Awesome

- Adds no-font-awesome variable to template to turn off remote FA
- FA is added in header as <script defer> for header buttons
- fa_icon() will include the rmarkdown FA dependency

R/bulma_document.R

       src = bulma_file("bulma"),
       stylesheet = "bulma.min.css"
     ),
-    rmarkdown::html_dependency_font_awesome(),
     if (toc) htmltools::htmlDependency(
       name = "gumshoe",
       version = "3.5.1",

inst/bulma/bulma.html

 $for(css)$
   <link rel="stylesheet" href="$css$" />
 $endfor$
-  <!--
-  <script defer src="https://use.fontawesome.com/releases/v5.3.1/js/all.js"></script>
-  -->
+$if(no-font-awesome)$
+$else$
+<script defer src="https://use.fontawesome.com/releases/v5.3.1/js/all.js"></script>
+$endif$
 $for(header-includes)$
   $header-includes$
 $endfor$