Added

• Added a resume feature when downloading aaxc files.
• New downlaoder module which contains a rework of the Downloader class.
• If necessary, large audiobooks are now downloaded in parts.
• Plugin command help page now contains additional information about the source of
  the plugin.
• Command help text now starts with ´(P)` for plugin commands.

Changed

• Rework plugin module
• using importlib.metadata over setuptools (pkg_resources) to get entrypoints