Set order of precedence for subtitle

Look first in the front-matter for the _index.md file, and secondly in
the parameters defined in the config.toml file.

layouts/index.html

@@ -2,8 +2,10 @@
 <main>
   <header class="homepage-header">
     <h1>{{.Title}}</h1>
-    {{ with .Site.Params.Subtitle }}
-    <span class="subtitle">{{.}}</span>
+    {{ if .Params.Subtitle }}
+    <span class="subtitle">{{.Params.Subtitle}}</span>
+    {{ else if .Site.Params.Subtitle }}
+    <span class="subtitle">{{.Site.Params.Subtitle}}</span>
     {{ end }}
   </header>
   <div class="homepage-content">