task #54: Template Directories for Debian package

After a second round, I think I managed to get everything working.
[This pullrequest](https://github.com/sbt/sbt-native-packager/pull/61) could then be closed without merging.

DSL would look like

```scala
linuxPackageMappings in Debian += packageTemplateMapping(target.value,
    "/var/log/" + name.value,
    "/etc/" + name.value
)
```