Add download dir and thread context options

[Clausinho]

Summary

• add --download-dir to store files outside the default downloads/board/thread path
• add --save-thread-context to save the current thread HTML and 4chan API JSON
• retry transient RemoteDisconnected errors instead of exiting the watcher

Smoke tests

• python3 -m py_compile inb4404.py
• python3 inb4404.py --help | rg -n "download-dir|save-thread-context|track-downloaded"
• fetched /b/ catalog and saved HTML/JSON for one live thread without downloading media
• fetched /gif/ catalog and saved HTML/JSON for one live thread without downloading media
• python3 thread-watcher.py -b b -q . -f -n smoke-test returned 150 catalog thread URLs
• python3 thread-watcher.py -b gif -q . -f -n smoke-test returned 76 catalog thread URLs