Add default for TMUX_CONF_DIR

.bash_tmux

@@ -1,6 +1,9 @@
 #!/bin/bash
 # vim: set filetype=sh:
 
+# tmux configuration files are found alongside this file
+export TMUX_CONF_DIR=${TMUX_CONF_DIR:-"$( dirname "${BASH_SOURCE[0]}" )"}
+
 function tmux_has_main() {
   tmux -2 has-session -t "Main Screen"
 }