add `handler` option to repl task

This commits allows users to specify a custom handler
function when running the repl task:

user=> (boot (repl :handler 'cider.nrepl/cider-nrepl-handler)))

The line above requires cider/cider-nrepl to be specified
as dependency. From the command line you can do that and
start the REPL like so:

boot -d cider/cider-nrepl:0.8.0-SNAPSHOT repl -x cider.nrepl/cider-nrepl-handler
Merge pull request #17 from martinklepsch/custom-repl-handler

add `handler` option to repl task