Major rewrite to adjust for the new response provided by YouTube

Porting code

request options for miniget were moved to options#requestOptions
nextpageRef was replaced by the continuation feature => check the README for more information
pages was added as a more request-efficient alternative to limit
playlist#visibility#link_only was renamed to unlisted
all thumbnails are now Objects of type Image featuring url, width and height properties - arrays of images are always sorted by descending resolution
all links have been renamed from ref to url to keep it consistend across all libraries
all snake_case variables were replaced by CamelCase
response#result_items is now response#estimatedItemCount
all users/channels feature both the channelID as well as a canonical url

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