Update: Define user variable to opt for a private project

Something I was planning to do since sometime — allow generating
private projects via `go-template`

Up until now, projects were generated by `go-template` under the
assumption that they will always be public — which, isn't always
true. Private projects would end up with broken links on their
README because shields.io can't reach the project!

This commit is the first step in this direction

Signed-off-by: notsatan <git@notsatan.me>

cookiecutter.json

@@ -6,6 +6,7 @@
     "base_branch": "master",
     "contact_email": "",
     "github_specific_features": "y",
+    "private_project": "n",
     "use_codecov": "y",
     "use_precommit": "y",
     "go_version": [