Move format to extra yaml prop to allow for file extension customization

[iDVB]

It would be helpful if this plugin allowed for...

custom:
  output:
    format: toml # toml, yaml, yml, and json format is available
    file: .build/.env

This way you can name the file anything you want.

Helps with trying to standardize with other plugins like this...
https://github.com/mrsteele/dotenv-webpack

For the moment, I'm simply relying on the above plugin's ability to look for a different name other than .env.

[ryan-mars]

@sbstjn Why hasn't this been merged?