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
Changes to repl task option handling

- The -m/--middleware option can take namespaced symbols. These are
  resolved by the repl-server code. These symbols may refer to vars that
  contain vectors of symbols which will also be resolved.

  See the cider nrepl middleware for an example:
  https://github.com/clojure-emacs/cider-nrepl/blob/v0.7.0/src/cider/nrepl.clj#L14-L25
Don't try to compose handler with middleware
2.0.0-pre23 -- REPL task enhancements
Change from hard links to soft links.