v1.7.0 - Plugin system, append windows to current session

This will be the last Python 2.7 release of tmuxp. Bug fixes for python
2.7 will live in the 1.7.x branch.

• #530 New feature: Plugin system

  • Add plugin system for user customization of tmuxp
  • Add tests for the plugin system
  • Update existing tests for the plugin system
  • Add the plugin interface to the tmuxp package
  • Add in depth documentation for the plugin system

  Thank you @joseph-flinn!

• #656 New feature: Ability to append windows to a session

  tmuxp load -a configfile will append a configuration to your current
  tmux session.

  Thank you @will-ockmore!

• #647 Improvement: Logging to file:

  tmuxp load <filename> --log-level outputfile.log

• #643 New command: Debug information

  Port tmuxp debug-info from via v1.6.2