Fixed comments & renamed Downloader.load_plugins to .find_plugins
wip implemented plugin loader and download methods

main.py:
  implemented Downloader.find_plugins,
    it searches ./plugins and loads up and modules it finds.
  Also implemented Downloader.download_file(),
    which does what it says on the tin.

plugins/nexusmods.py:
  Minor refactoring, mainly moved the can_handle attribute
    inside the plugin class. Using class methods to query
    a plugin about its capabilities is much cleaner than
    grabbing a bunch of module-level vars and stuffing
    them into an half-assed data structure.

  Also made the plugin attempt to login if it notices that
    it isn't, instead of just giving up and throwing an
    exception.

(yay, hanging indents)