add support for nrepl-handler (e.g. cider-nrepl)

I wanted to use cider-nrepl with clojure-maven-plugin, so I added support for [handler](https://github.com/clojure/tools.nrepl#handlers) in the nrepl mojo. To use it with cider-nrepl just configure `nreplHandler` or the property `clojure.nrepl.handler` to `cider.nrepl/cider-nrepl-handler` and add `cider-nrepl` to the projects dependencies.