WIP: Rewrite

A work-in-progress rewrite. While the branch is in usable state, there still are things that need to be done before it's released:

• MAJOR link shortening
• MAJOR migration guide
• MAJOR notifications for success and fail
• response parsing that doesn't depend on jq
• optional persistent screenshot store
• better locking for gif recording
• macOS gif recording (?)
• macOS workflow
• other recorder formats
• support for no /tmp
• stdin by default
• document more examples in manpage
• hash verification