Added tmux session chooser. Aliased to `tm`, calling `tmux_attach_start`
(the original tm script).
When called without arguments displays a fzf list of currently running
tmux sessions, with a preview to their respective open panes. A session
can be chosen in fzf which tmux will attach itself to. When creating a
query in fzf which does not have a valid target and confirming, tmux
will automatically create that session and attach itself to it.
When called with an argument, tmux will attach itself or create a
session of the same name. If called with the name of a session file, as
before, tmux will automatically execute that session file and attach
itself to it.
