Exposed browser-repl-env for use with vim

Vim-fireplace's `:Piggieback <env>` command will start a Clojurescript
repl with the provided env, so have exposed browser-repl-env.

Also added documenation on using with Vim.
Merge pull request #3 from leafgarland/vim-fireplace-compatibility

Documentation and changes to make this work with vim and vim-fireplace