Add tests for line blocks handling.
Overhaul line blocks.

Line blocks used to be parsed line-by-line, which made emphasis
spanning across lines, using blockquotes inside of line blocks,
etc. impossible. Now their content is parsed with standard block
parser with `stateHardBreaks` set.
Fix inline code elements with line breaks in Markdown.
Update README for line block handling changes.
Add type signature to `lineBlockTests`.