Allow manual URL list to be passed to crawler

[stooit]

This adds an option to the crawler to parse a file containing a list of URLs to add to the queue.

Usage: quant crawl --urls-file=/path/to/file.json

Where file.json is a simple array of absolute URLs:

[
  "https://www.google.com/url-a",
  "https://www.google.com/url-b"
  ...
]

This is useful for allowing orphaned URLs to be added to ongoing crawl operations.