Consider allowing DEST_DIR be set to in [nodeenv] config section

[markjm]

Hi!

Thank you for the great project. I am wondering if you would be willing to accept a PR for setting DEST_DIR in the configuration settings?

I have a dockerized project which I use nodeenv in. Since its dockerized, we (for better or worse) dont use a venv, so rely on passing DEST_DIR on the commandline. Instead of aligning everywhere which may use nodeenv, I figure it may be valuable to just set it in config.

Any thoughts? If you are happy to have this option, I can make the change!