make build_home_index() use content-home template (#787)

[goldingn]

As discussed in #787: the content-home.html template was being ignored when a .Rmd file was used for the index page content (ie. index.Rmd or README.Rmd, as opposed to index.md or README.md). Instead, the .Rmd was rendered as an article.

This PR:

• adds tests that the template and content are used for generating index.html.
• fixes the bug by generalising rmarkdown_template() to take names other than 'article', and passing the Rmarkdown content through to pandoc by substituting the whisker variable {{index}} with the pandoc variable $body$

The two tests that are added in this PR are successful, but there are two failing tests (and two other warnings) on the master branch already, so I expect the travis checks to fail.

[jayhesselberth]

These changes caused some formatting issues in the Installation code block on the pkgdown site.

The diff suggests there are several indentations introduced by the content-home template.

[hadley]

@jayhesselberth do you think this is good to merge?

[jayhesselberth]

Looks good to me.

[goldingn]

Great! Looks like appveyor is having trouble installing some packages though.

---

Relatedly https://greta-stats.org is a custom pkgdown build with an Rmd home doc, and a custom home template (with quite a lot of html & css fiddling). It was built using this branch, but I'd be very happy for it to be an example on the pkgdown home page once this is merged. If you're keen, let me know and I'll open a PR to add a line to the pkgdown README.Rmd.

[hadley]

Can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).

(Then I'll merge)

[goldingn]

Done