Allow tmux session name to be passed as env var

For normal usage this change should be a NOOP. However it enables the
use case where the user might want to predefine what session gets
connected to when the shell is launched. Running `zsh` will still net
you a connection to the last used tmux session or a new one named
"zprezto" if none exists. However `env tmux_session=myname zsh` will
connect you specifically to a session called 'myname' including creating
it if it doesn't exist.

This is particularly useful in window managers where a key binding might
open a terminal with a predefined session or even show a list of current
sessions and open a new terminal connected to that session.

The default session name may still be set using zstyle as documented.

modules/tmux/init.zsh

@@ -29,16 +29,17 @@ if [[ -z "$TMUX" && -z "$EMACS" && -z "$VIM" && -z "$INSIDE_EMACS" && -z "$VSCOD
 ); then
   tmux start-server
 
-  # Create a 'prezto' session if no session has been defined in tmux.conf.
-  if ! tmux has-session 2> /dev/null; then
-    zstyle -s ':prezto:module:tmux:session' name tmux_session || tmux_session='prezto'
+  # Make sure we have a session to connect to, either named via an environment
+  # variable, the default style as document, or 'prezto' for last resort
+  [[ -z "${tmux_session:// }" ]] && zstyle -s ':prezto:module:tmux:session' name tmux_session || : ${tmux_session:=prezto}
+  if ! tmux has-session -t "$tmux_session" 2> /dev/null; then
     tmux \
       new-session -d -s "$tmux_session" \; \
       set-option -t "$tmux_session" destroy-unattached off &> /dev/null
   fi
 
-  # Attach to the 'prezto' session or to the last session used. (detach first)
-  exec tmux $_tmux_iterm_integration attach-session -d
+  # Attach to the session name requested in an environment variable or the default (detaching others)
+  exec tmux $_tmux_iterm_integration attach-session -t "$tmux_session" -d
 fi
 
 #