Show task updates for attachments, labels, and a recent stories list

- The way attachments are represented by the API, when a note had attachments, the full note was printed out with each one in the attachment section of show_task. This update folds in attachments so they appear with the note they are attached to.

- This update saves a list of recently shown tasks in the local config and lists them with "pt recent"

- Also 'pt label' command to add labels to a story