Frontmatter tag format

Frontmatter tags should be a list (in brackets). If I'm going to do
this, I may as well do it right.

.vim/templates/mdfm

@@ -3,7 +3,7 @@
 filename: 
 filecreated: 
 fileupdated: 
-filetags: 
+filetags: []
 ---