Add maktaba#plugin#Register that never touches runtimepath

Maktaba as a plugin framework should allow plugins to register/init themselves via maktaba without side effects like modifying runtimepath that should generally be left up to plugin managers. This change adds a `maktaba#plugin#Register` function to do just that. Also changes all `maktaba#plugin#GetOrInstall` calls in maktaba to `maktaba#plugin#Register` because:
  * in many cases like `maktaba#plugin#Detect` and `maktaba#plugin#Enter`, we can assume that the plugin is already on runtimepath and skip the overhead of checking, and
  * maktaba as a library shouldn't be modifying runtimepath unless instructed to by a plugin manager or explicit calls to `maktaba#rtp` utilities.

Resolves #50.