Allow using '$HOSTNAME' in @resurrect-dir

Closes #197

docs/save_dir.md

@@ -4,3 +4,12 @@ By default Tmux environment is saved to a file in `~/.tmux/resurrect` dir.
 Change this with:
 
     set -g @resurrect-dir '/some/path'
+
+Using environment variables or shell interpolation in this option is not
+allowed as the string is used literally. So the following won't do what is
+expected:
+
+    set -g @resurrect-dir '/path/$MY_VAR/$(some_executable)'
+
+Only the following variables and special chars are allowed:
+`$HOME`, `$HOSTNAME`, and `~`.