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

@@ -1,9 +1,9 @@
 :insert
 ---
-filename:
+filename: 
-filecreated:
+filecreated: 
 fileupdated: 
-filetags: 
+filetags: []
 ---