Minor cleanups after #62

[domenic]

Opening this so we don't forget.

• Stop copying things into $HTML_CACHE, and just use them directly
• Add a small section to the top-level README.md explaining what the quotes/ and entities/ directories are about.
• Stop using HTML_* environment variables, pass arguments instead. (a natural part of "stop copying")
• Figure out how to monitor for changes to import to quotes/ and entities/

... anything else?

[foolip]

(contents moved into the OP so that we get that nice 0/4 tracking from GitHub)