Introduce an 'initialize()' function that does the actual init
(session, cookies, config options) and can called separately from
the constructor __init__().
This allows, for example, to adjust config access inside a Job
before most of it already happened when calling 'extractor.find()'.
Instead of using a refresh-token-based access-token for every API
request, they are now only used for paginated results.
API requests to get a user's profile and the original download URL
now always use a public access-token.
- simplify regex
- unquote search tags
- increase default wait-time between HTTP requests
- downloading several hundreds of images always resulted
in '429 Too Many Requests' eventually
- circumvent paging restrictions for unauthenticated users by only
using the 'next' parameter
- setting 'page' to a constant, low value (or simply omitting it)
does the trick
