Update shell to create new page/post files

- bin/shell: update create page/posts commands
- bin/shell: fix loadPost() to process excerpt only if needed
- bin/shell: fix loadPost()/loadPage() to accept all markdown file extensions (markdown, mdown, mkdn, mkd, md)
- app/models/page.js: set image to null by default
- app/models/post.js: set image/excerpt to null by default
- content/pages/: add directory
- content/posts: add directory

Signed-off-by: Boris HUISGEN <bhuisgen@hbis.fr>