Make it easier to create multiple assemblies

[henriklundahl]

IIRC it's easy to create multiple assemblies with the Maven Assembly Plugin. With lein-assembly you would need to configure the different assemblies in different profiles and then use lein with-profile xyz assemble. Perhaps another level could be added to the configuration like for example

:assemble {:assembly-1 {:filesets {...}
                        ...}
           :assembly-2 {:filesets {...}
                        ...}}