Making inclusion of partial templates use the 'partial' function so that they can be overridden.

Hi - thank you for porting this theme to Hugo!

This patch updates the partial template inclusion to the [recommended method from version 0.12](http://gohugo.io/templates/partials/) so that it can do overrides from the local layout. 
Automated change with:

     find -name '*.html' |xargs grep -l theme/partials |xargs -L1 sed -i "s/template \"theme\/partials\//partial \"/g"