Allow inline html in _index.Rmarkdown (#42)

Starting with hugo 0.60, the default markdown renderer changed from
blackdown to goldmark. By default, goldmark does not render raw HTML so
specifying raw html in _index.Rmarkdown (in this instance to customize
an image's display) created an html file with <!-- raw HTML omitted -->
rather than the correct html to display the image.

You can make goldmark include raw html by switching it to `unsafe` mode
in the `config.toml` which this commit does.

As an FYI, An alternative is to mv `_index.Rmarkdown` to `_index.Rmd`
which will switch the markdown engine from goldmark to pandoc which
still renders raw html.

exampleSite/config.toml

@@ -34,3 +34,8 @@ footnotereturnlinkcontents = "↩"
 [params]
     description = "A website built through Hugo and blogdown."
     footer = "© [Yihui Xie](https://yihui.name) 2017 -- 2019 | [Github](https://github.com/yihui) | [Twitter](https://twitter.com/xieyihui)"
+
+[markup]
+  [markup.goldmark]
+    [markup.goldmark.renderer]
+      unsafe = true