Add :do task special form

* The :do special form can be used when defining tasks in :tasks in the
  boot.edn, for example:

  {:project ...
   :version ...
   :dependencies ...
   :require-tasks #{[tailrecursion.boot.task :refer :all]}
   :src-paths #{"src"}
   :tasks
   {:dev
    {:doc "Build for development."
     :main [:do [:watch] [:cljs {:optimizations :advanced}]]}}
Merge branch 'master' of gh:tailrecursion/boot.core