Update meta markdown snippets

The original snippet use vim interpolation and the date keeps changing while we are inside the
snippet region, which is not desired. According to https://www.youtube.com/watch?v=JJQYwt6Diro,
we can use Python interpolation and check if snip.c  is empty to see if the interpolation has been finished.
If the interpolation has finished (snip.c is empty in this condition), the date will be fixed and does not change
when we are inside the snippet region.

my_snippets/markdown.snippets

@@ -18,7 +18,9 @@ endsnippet
 snippet "meta(data)?" "Markdown metadata front matter" br
 ---
 title: "$1"
-date: `!v strftime("%Y-%m-%d %H:%M:%S%z")`
+date: `!p from datetime import datetime
+if not snip.c:
+	snip.rv=datetime.now().strftime("%Y-%m-%d %H:%M:%S%z")`
 tags: [$2]
 categories: [$3]
 ---