Add maktaba#plugin#Detect to register plugins and trigger instant/

Adds a new function, `maktaba#plugin#Detect()`, that should be called after all maktaba plugins have been installed to register them with maktaba and trigger them to execute any instant/ hooks.

Also adds automatic detection to `maktaba#plugin#RegisteredPlugins()`.

Note that I still decided not to add any fancy caching like I mentioned previously, because maktaba never really scans paths a second time and won't be incurring any significant overhead. The plugins are registered by the first `#plugin#GetOrInstall()` call and subsequent `#GetOrInstall()` calls become a cheap lookup.

Fixes #38.