initial powerloader integration

[wolfv]

cc @JohanMabille this is some very initial work on integrating with powerloader.

What's missing:

• mirror configuration
• auth config?
• subdirdata also via powerloader
• replace mamba::url with powerloader::url
• Decide if we want to use std::function or raw function pointers for callbacks
• Add mod / etag functions to powerloader to retrieve cached information
• Figure out if we should enhance the callback on powerloader side with throttling and average speed computation

lots of other things likely, too :)

[wolfv]

Also needs this powerloader PR: mamba-org/powerloader#79