Completions hook, flush after eval and internal refactoring

This has been sitting on develop for quite a long time now! The only noticeable change is that Conjure will flush upon eval now, that means you can use (print "foo") and see "foo" printed to *out* right away. Before this you would have to wait until a newline was printed or (flush) was called. This was only ever a problem in JVM Clojure, not ClojureScript, that already flushes automatically.

I've also added a :completions hook that allows you to map, filter or sort the completions as they're returned from Conjure before your Neovim completion plugin gets hold of them. This can be useful to filter out keys you don't care about.

There's been a number of internal source files that have been moved around to tidy up the top level directory, although that shouldn't be noticeable by any users.