HTML reader:  adjust `blockTags` and `eitherBlockOrInline`.

- Added `audio` and `source` in `eitherBlockOrInline`.
- Moved `video`, `svg`, `progress`, `script`, `noscript`, `svg` from
  `blockTags` to `eitherBlockOrInline`.
- `map` and `object` were mistakenly in both lists; they have been removed
  from `blockTags`.
normalize:  consolidate adjacent RawBlocks when possible.
`Parsing`: Added `stateInHtmlBlock` to `ParserState`.

This is used to keep track of the ending tag we're waiting
for when we're parsing inside HTML block tags.
EPUB writer:  better handle HTML media tags.
Revamped raw HTML block parsing in markdown.

- We no longer include trailing spaces and newlines in the
  raw blocks.
- We look for closing tags for elements (but without backtracking).
- Each block-level tag is its own RawBlock; we no longer try to
  consolidate them (though `--normalize` will do so).

Closes #1330.