Behavior of `set_key` on existing but commented variable.

[flowgunso]

The current behavior of set_key is to append to the env file the key/value pair even though an key already exists but is commented.

I would like to contribute such feature to uncomment an available key and update it, if you would be OK with that @bbc2 @theskumar ?

Obviously, this would change the current behavior of dotenv and it shall be an option.