Jekyll uses Redcarpet by default for translating Markdown to HTML, but

Redcarpet doesn't handle Markdown in HTML blocks (like div's).  This
PR switches to Kramdown, which *does* handle md-in-html.  However,
Kramdown translates single and double quotes into curly quotes by
default, which confuses downstream tools, so we have to turn that off.
While we're there, we also tell it to output character entities as
character entities.

_config.yml

@@ -1,4 +1,7 @@
 markdown        : kramdown
+kramdown        :
+  entity_output : as_input # — is output as —
+  smart_quotes  : [39, 39, 34, 34] # decimal for left/right single/double quotes
 pygments        : true
 contact         : "info@software-carpentry.org"
 github_url      : "https://github.com/swcarpentry"