includeDate returns `null`

Just a heads up:
```sh
$ node -e "console.log(require('git-repo-version')({ shaLength: 10, includeDate: true }))"
0.0.1+aceae13b5b null
$ node --version
v9.4.0
$ sw_vers
ProductName: Mac OS X
ProductVersion: 10.13
BuildVersion: 17A365
```

Reproduce the Node.js command shown in the issue and trace how the includeDate option is handled. Establish the intended date representation from the project context before changing behavior, then add coverage for this case; done means includeDate no longer produces null according to that defined behavior.

Written by the indexing model from the issue text.