Only process *.md files in posts

The editor may leave behind backup files or worse, have a swap file in
the directory due to the post file being edited.  shonku currently
processes it and may even generate a separate post for it under some
conditions.  This patch fixes that by making shonku only process *.md
files and have rsync only sync *.md files in the posts directory.

Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>