Use .Description for meta description if available

[edruid]

Background
When indexing my site with Bing it told me that the descriptions for my pages were too short on almost every page.

Proposed solution:
Change themes/geekdoc/layouts/partials/head.html to use .Description with partial "title" . as falback (there is probably a more elegant way to this but...):

<meta name="description" content="{{ if .Description }}{{ .Description }}{{ else }}{{ partial "title" . }}{{ end }}">