Support Class-Path in jar manifest

[lilyevsky]

Currently the jar task does not support the Class-Path attribute.

This attribute is one of the general main jar attributes, see https://docs.oracle.com/javase/8/docs/technotes/guides/jar/jar.html#Main_Attributes .

The Class-Path is very important for standalone apps that are deployed as a collection of jar files, the main jar and dependent libraries (not as uber-jar).

[lefou]

You can overwrite the def manifest: T[JarManifest] target and append any manifest entry you want, including Class-Path.