Add smarty compile id to prevent collisions when using a single cache dir

The plugin instance function is using a single compile directory for multiple template directories. A [compile id][ci] will prevent collisions within this directory.

This manifested itself this morning for me when viewing the summary (view templates) in multiple plugins. The cached template from the first was being used to render the info for the second. I believe this should fix this problem.

[ci]: http://www.smarty.net/docsv2/en/variable.compile.id.tpl