Simulate background-clip: text for browsers that don't support it

Move the heading generating to a partial that iteratively wraps each character
of the heading in a <span>. The CSS for the site heading uses :nth-child to
color each span a different color.

layouts/partials/site_name.html

@@ -0,0 +1 @@
+{{ range (split . "") }}<span>{{ . }}</span>{{ end }}