don't clober .env.example

[ofbeaton]

Please either add a flag to omit generating the .env.example file, or better yet read the current example file and don't change variables that already exist.

For example in my .env.example file I have

DB_TYPE=postgres

but after running gen-env, it contains

DB_TYPE=

and so is no longer an accurate example.

And while this doesn't affect me personally, dotenv and dotenv-save allow you to use a custom .env filename, so you may want to make this configurable in this tool as well (unless -e already does this?)