Optionally manage the default license, author, and email for new extensions.

Example usage:

```
civix config:set email info@civicrm.org
civix config:set license AGPL-3.0
```

If no value has been set, then fallback to reading "git config" or using
hard-coded defaults.
Merge pull request #32 from totten/master-license-defaults

Optionally manage the default license, author, and email for new extensions.