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Add functions to attach tmux sessions, reload env

Add functionality that:

- Add file to create a tmux session or reattach to an existing one.
- Implement the reloading of the environment from the tmux session upon
  each reattach (if done using
  bin/__tmux-session-with-reattach-callback).
Merge remote-tracking branch 'origin/master'
Merge branch 'master' of github.com:telotortium/dotfiles