cmd/end.js :: use reltime to change --when date was

	--when "-34m" # I was done with this 34 minutes ago
lib/vim.js :: pass `done` callback to our handler to issue fs.unlink

This allows the consumer to only erase the file if they successfully
handled the contents we've given them.
cmd/begin.js :: use reltime to modify date and use vim's done argument
lib/vim.js :: allow first arg to be text for editing
cmd/update.js :: allow users to edit their msg offline
README.md :: users can now `update` their last start msg offline