Added {thumbnail} and {featuredimage} template tags

Added two tags for use in the publish format template:
{thumbnail} grabs the featured image thumbnail for the post
{featuredimage} grabs the full-size featured image for the post

Note: Using {featuredimage} may or may not need to have "create_thumbnails" enabled in discourse for best results

An example of how this comes out can be seen here:

http://forums.ctrlcmdesc.com/t/costume-quest/298
Merge pull request #29 from surrealroad/patch-6

Added {thumbnail} and {featuredimage} template tags