Expose the configuration options to the user

[Kyle-Falconer]

The program uses options which are currently hard-coded. These should all be settable by the user:

• depth_limit : how deep down the rabbit hole should we go?
• fail_tolerance : how many times should a failed link be retried?
• threading_method : use the cached thread pool or fixed thread pool
• max_thread_limit : if fixed thread pool is selected, this defines how many threads to spawn
• user_agent : what user agent to use when making the page request
• referrer : what referrer to use when making the page request