Sessions as tmux windows

[suvayu]

Integrate with tmux to implement search sessions.

Example use

• A user is searching based on a class name. They find the relevant
  file, but now they need to search for different token without
  loosing the current status.

• Start a fresh search session from the original location/current
  directory (TBD) in a new tmux window.

Notes

When tmux is present, every session is in a tmux window, ending a
session closes the tmux window. If already in tmux, maybe don't
nest tmux sessions.