Add a hook that runs on load-file completion

The hook allows nrepl.el clients such as ritz and clojure-test-mode to
trigger actions when a file is loaded into the repl session.
Merge branch 'feature/add-file-loaded-hook' of https://github.com/hugoduncan/nrepl.el into hugoduncan-feature/add-file-loaded-hook
Fix checkdoc.