Add hideReply functionality to pages

Set the `hideReply` param on front matter to completely remove the "reply
to" link present in the bottom of the pages. This is
backwards-compatible given that any page that does not have the
`hideReply` param will continue to work as before.

layouts/_default/single.html

@@ -16,6 +16,7 @@
     <a class="blog-tags" href="{{ .RelPermalink }}">#{{ lower .LinkTitle }}</a>
   {{ end }}
 </p>
+{{ if not .Params.hideReply }}
 {{ with .Site.Params.author.email }}
   <p>
     <a href='mailto:{{ . }}?subject={{ i18n "email-subject" }}"{{ default $.Site.Title $.Page.Title }}"'>
@@ -24,3 +25,4 @@
   </p>
 {{ end }}
 {{ end }}
+{{ end }}