Changes since 6.5.5:
* Add knob to invoke folderfilter dynamically on each sync (GitHub#73)
* Add knob to apply compression to IMAP connections (Abdó Roig-Maranges)
* Add knob to filter some headers before uploading message
  to IMAP server (Abdó Roig-Maranges)
* Allow to sync GMail labels and implement GmailMaildir repository that
  adds mechanics to change message labels (Abdó Roig-Maranges)
* Allow to migrate status data across differend backends
  (Abdó Roig-Maranges)
* Support XDG Base Directory Specification
  (if $XDG_CONFIG_HOME/offlineimap/config exists, use it as the
  default configuration path; ~/.offlineimaprc is still tried after
  XDG location) (GitHub#32)
* Allow multiple certificate fingerprints to be specified inside
  'cert_fingerprint'