Add flags for returning different post formats on publish

[thebaer]

Overview Allow users to get a publish URL that reflects the format they want, e.g. plain text or with rendered Markdown.

Background Appending certain extensions to anonymous post URLs makes them render differently. E.g. .txt returns a full post as text/plain and .md renders Markdown in a post.

Implementation Upon successfully publishing a new post, simply append the correct extension to the write.as URL based on flags given. Plain text URLs end in .txt, e.g. https://write.as/ql9ed0uc49pdp1qu.txt, and Markdown URLs end in .md, e.g. https://write.as/ql9ed0uc49pdp1qu.md

Correct solution

• Support new --format flag (or a better name) that accepts raw or md/markdown -- e.g. --format raw

Original solution

• Passing --raw should return the post URL ending in .txt
• Passing --markdown should return post URL ending in .md

[thebaer]

Moved to Phabricator T587.