Adding default GitHub .gitignore for Go projects

to, among other things, avoid accidentally committing the `*.test` files
Adding support for alternate variable names
Adding `accept_smushy_name` tag to not breaking backwards compatibility

Basically, if the user wants to use an alternative name going forward,
adding the `accept_smushy_name:"yes"` tag will ensure that the old env
vars still work while allowing the alt ones to take precedence.