Add entrypoint function which returns a repl-env.

To use this wonderful cljs REPL with [fireplace.vim](https://github.com/tpope/vim-fireplace), I have added a function in the same style as `start-repl`, called `repl-env`.

Fireplace has a `:Piggieback` command that given a usable REPL environment invokes `cemerick.piggieback/cljs-repl` with that environment. Thus, `start-repl` is not possible to use with fireplace for two reasons:

1. It invokes piggieback directly, thereby starting a REPL
2. It does not return a usable REPL environment

It does not make sense to let `start-repl` return the environment, since a REPL is already started on it; and it certainly does not make sense to remove the piggieback invocation. For those two reasons I decided to add a new function that does neither to begin with, for use in places that expect to invoke piggieback themselves. Furthermore I have extracted the functionality that is shared among the `start-repl` and the new `repl-env` functions into separate functions to reduce duplication.

I figured a PR to this task was better than creating a new one, since almost all of the code is shared among the two.