Add hook/plugin system

[NoahTheDuke]

inspired by kaocha's hooks, here's a poor man's version.

i've been poking at this since december, i'll probably merge it soon but i'm not sure yet.

(defhook yell
  "Prints a message at the start and end of the whole run."
  (pre-test-run
    [config m]
    (println "STARTING")
    m)
  (post-test-run
    [config m]
    (println "ENDING")
    m))

it works, it's pretty chill, it doesn't seem to slow down the runner much.