[#63] Re-add the tmux plugin

This reverts commit da7c67d1da.

plugins/tmux/init.zsh

@@ -0,0 +1,33 @@
+#
+# Defines tmux aliases and provides for auto launching it at start-up.
+#
+# Authors:
+#   Sorin Ionescu <sorin.ionescu@gmail.com>
+#
+# Usage:
+#   To auto start it, add the following to zshrc:
+#
+#     # Auto launch tmux at start-up.
+#     zstyle -t ':omz:plugin:tmux:auto' start 'yes'
+#
+
+# Aliases
+alias ta="tmux attach-session"
+alias tl="tmux list-sessions"
+
+# Auto Start
+if (( $SHLVL == 1 )) && zstyle -t ':omz:plugin:tmux:auto' start; then
+  (( SHLVL += 1 )) && export SHLVL
+
+  session="$(
+    tmux list-sessions 2> /dev/null \
+      | cut -d':' -f1 \
+      | head -1)"
+
+  if [[ -n "$session" ]]; then
+    exec tmux attach-session -t "$session"
+  else
+    exec tmux new-session
+  fi
+fi
+