Extra Classpath

I added another setting to add additional files/folders to the classpath:
```scala
packExtraClasspath: Map[String, Seq[String]]
```

I use it to put configuration files inside "src/pack/etc" and configure the project as follows:
```scala
packExtraClasspath := Map("app" -> Seq("${PROG_HOME}/etc"))
```
to have the configuration files inside the classpath of the application to avoid reading them directly from the filesystem.