Check if commentoUrl is set before using it. (#198)

* Check if commentoUrl is set before using it.

When a variable does not exist it evaluates to `nil`, and so the
check `(not (eq .Site.Params.commentoUrl ""))` evaluates to true rather
than false because `nil` is not "".  Before this change, unless you
explicitly set Params.commentoUrl to "", the commento HTML block would
be added, and this fails because /js/commento.js does not exist.  This
change ensures that the commento HTML block is only added whe
Params.commentoUrl exists and is set to a non-empty string.

* Check if DisqusShortname is set before using it.

layouts/partials/posts/commento.html

@@ -1,4 +1,4 @@
-{{- if and (not (eq .Site.Params.commentoUrl "" )) (eq (.Params.disable_comments | default false) false) -}}
+{{- if and (isset .Site.Params "commentourl") (not (eq .Site.Params.commentoUrl "" )) (eq (.Params.disable_comments | default false) false) -}}
 <div id="commento"></div>
 <script src="{{ .Site.Params.commentoUrl }}/js/commento.js"></script>
 {{- end -}}

layouts/partials/posts/disqus.html

@@ -1,3 +1,3 @@
-{{- if and (not (eq .Site.DisqusShortname "" )) (eq (.Params.disable_comments | default false) false) -}}
+{{- if and (isset .Site "disqusshortname") (not (eq .Site.DisqusShortname "" )) (eq (.Params.disable_comments | default false) false) -}}
   {{ template "_internal/disqus.html" . }}
 {{- end -}}