alpha-0.6.0

- Add `select` command to select threads that match search query

- Add support for mass copying/deleting/flagging/moving of messages
- IMAP: add support for COMPRESS=DEFLATE and others
  Extension use can be configured with individual flags such as `use_deflate`
- Rename EXECUTE mode to COMMAND
- add async IMAP backend
- add in-app SMTP support
- ui: Show decoded source by default when viewing an Envelope's source
- ui: Add search in pagers
- Add managesieve REPL binary for managesieve script management
- imap: `add server_password_command`
- configuration: Add per-folder and per-account configuration overrides.
  e.g. `accounts."imap.domain.tld".mailboxes."INBOX".index_style = "plain"`

  The selection is done for a specific field as follows:

  ```text
  if per-folder override is defined, return per-folder override
    else if per-account override is defined, return per-account override
      else return global setting field value.
  ```
- themes: Add Italics, Blink, Dim and Hidden text attributes
- ui: recognize readline shortcuts in Execute mode
- ui: hopefully smarter auto-completion in Execute mode
- demo NNTP python plugin
- ui: add `auto_choose_multipart_alternative`: Choose `text/html` alternative if `text/plain` is empty in `multipart/alternative` attachments.
- ui: custom date format strings
- ui: manual refresh for mailbox view
- ui: create mailbox command
- fs autocomplete
- ui: add support for [`NO_COLOR`](https://no-color.org/)
- enhanced, portable Makefile
- added Debian packaging
- added `default_header_values`: default header values used when creating a new draft
- ui: switch between sidebar and mailbox view with {left,right} keys for more intuitive navigation
- ui: add optional filter query for each mailbox view to view only the matching subset of messages (for example, you can hide all seen envelopes with `filter = "not flags:seen"`

- Replace any use of 'folder' with 'mailbox' in user configuration
- Load libnotmuch dynamically
- Launch all user shell commands with `sh -c "..."`

- notmuch: add support for multiple accounts on same notmuch db