Create token creation command

Coming from #83

Create the `$GREN_GITHUB_TOKEN` using the [API to generate a personal access token](https://help.github.com/articles/creating-a-personal-access-token-for-the-command-line/)

Start by reading issue #83 and the linked GitHub documentation on creating a personal access token. Trace the existing command entry point and token handling, then verify that the requested command creates or guides setup of GREN_GITHUB_TOKEN with the intended permissions.

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