Escape content of html code block by default

[tlienart]

• This is also what is done in the default in Julia's Markdown
• Remove the FAQ element with the esc from the docs
• Corner case is @def lang = "html" then all code blocks should be escaped
• This is (midly) breaking so that should go with a minor release.
• Will break docs in JuDocWeb.

Not very high priority...

Edit: could do it in backward compatible way by checking whether there's any < or > and not acting if there is...