Scrape links from the thread comments

chanarch.py (__main__):
  - Add linkfile argument, which will non-destructively add any URLs in
    the thread to the linkfile

chanarch.py (ChanThread):
  __init__():
    - Add linkfile argument

  set_thread():
    - Add linkfile argument

  scrape_links():
    - New method
    - Scrape the links from thread comments into a file

README.rst:
  - Explain thread link scraping arguments