CLI to make authoring easier

[robinmetral]

I really like how Cobalt has a CLI to author posts:

• cobalt new creates a new post using a given template
• cobalt publish sets a post's is_draft to false and sets the published_date to the current date
• (not about authoring pages per se: cobalt init initializes the Cobalt directory structure)

Other ideas:

• update: add or update the updated_date
• 🚧

This is possible to implement when there are well-defined conventions for directory structure and post metadata (frontmatter), something that more flexible generators cannot do.