Add support for Nashorn & Node.js repls.

[levand]

Code for all three "embedded" REPLs is now in a single namespace.

This required upgrading the Clojure and ClojureScript versions used for development and testing, which also required upgrading Midje and lein-midje.

Test updates are included, though I could not get the tests to run properly in the plugin project - I am not sure how they could run given the current state of the code. Midje seems to bail early instead of acknowledging an expected error. Tests in support pass correctly.

Documentation is also included

[crocket]

I want this to be merged!!

[danielcompton]

@crocket this kind of comment comes across as unhelpful. Everybody working or contributing on cljsbuild (and most open source projects) are doing it as a gift for the community. If you're interested in seeing it merged, then a polite :+1: would probably suffice. Open source burnout is a real thing, and these kinds of comments contribute to it.

[mneise]

@levand Happy to merge this. I don't mind doing a manual merge to resolve the conflicts, or would you want to resolve the conflicts yourself?