Support author name in LicenseHeaderStep

Some license texts include the name of the file's initial creator. This is currently not possible to do with the `LicenseHeaderStep`.
Maybe the step could should work with templates rather than constant strings, e,g,
```groovy
licenseHeader('/* Copyright ${year} ${author}*/',
[year: java.time.LocalDate.now().year, author: 'John Doe'])
```