Checkout task

[micha]

This task adds the ability to add "checkout dependencies", similar to what Leiningen has.

In boot it's done a slightly different way, though. You'll need to run two separate invocations of boot: one to build the checkout dependency and another for your project.

The checkout build, for example:

$ boot watch pom jar install

and the project build:

$ boot repl -s watch checkout -d acme/foo:1.2.3 cljs-repl reload cljs serve