Add default renderers for html nodes pointing at the README

[borkdude]

Addresses #69.

Parsing was already CommonMark compliant. <del>\n\n*foo*\n\n</del> (https://spec.commonmark.org/0.30/#example-167) parses to html-block, paragraph, html-block, and the <aside> case in the issue matches what markdown-it produces. What made HTML look broken is that default-hiccup-renderers had no entry for :html-block and :html-inline, so ->hiccup fell through to the generic "Unknown type" span.

• Default renderers for :html-block and :html-inline that say there is no default renderer and link to the README section.
• README: fix the :html typo, and document that an HTML block ends at the first blank line.
• Tests for the new message and for example 167.
• Drop a duplicated pair of html_inline / html_block defmethods in impl.cljs.

Rendering raw HTML by default is still not the behaviour, since that means unescaped passthrough.

[borkdude]

@zampino This looks like a reasonable thing to want so we don't get issues reported anymore like #69 - WDYT?

[zampino]

@zampino This looks like a reasonable thing to want so we don't get issues reported anymore like #69 - WDYT?

sounds good