Add adzerk.boot-cljs-repl task inline.

Initial experiments with extracting a usable repl-env
for use with `:Piggieback`.
Successfully modified adzerk.boot-cljs-repl for fireplace.vim.

By removing the call to piggieback made by the original `start-repl`
function fireplace.vim's `:Piggieback` command is allowed to take
full control of the returned weasel environment.