list_external_links transformed into generic find_in_all_posts
dump_json() genericized, filename now choosable in find_in_all_posts()
new download_json() function, restored dump_json() function
comment clarification
Merge branch 'master' of https://github.com/bibanon/BA-4chan-thread-archiver

* 'master' of https://github.com/bibanon/BA-4chan-thread-archiver:
  adding upgrade instructions
  updating for new 4chan URL changes
  updated for latest 4chan URL changes
  Update README.rst
  Update README.rst
fixed linkregex typo, fixes to UTF-8 bugs
gitignore additions