move credentials out of main GitHub config template, or at least use variables

Ref. [templates/config:81](https://github.com/mkwmms/ansible-github/blob/1781a2ff6818d4d061c55750b4dda47090e2c6ff/templates/config#L81)

```
# TODO: move credentials out of main GitHub config template, or at least use variables

#####=== Credentials ===#####

; [github]
;   user = mkwmms
…
```
