release-2.7

This release features a few small additions and tweaks to our behaviour:

• Show the time when sleeping is beginning
  • Implemented by @clh021 in #93.
• Added the Content-Base header to generated eamils
  • Implemented by @StayPirate in #96.
• Minor fixes to internal documentation and CI-system
  • Largely implemented in #87.
• Added the unsee sub-command.
  • This removes a single entry from our history, and was implemented in #88.
  • Later this was updated to allow a regular expression to be specified instead of a literal match.
• Added the env and split functions to the template environment.
  • Reported in #97, implemented in #98.
• Allow a "tag" item to be set for each source-feed, and used in the email template.
  • Reported in #94, implemented in #99.