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, which is a new flag in `ParserState` instructing end-of-line parser to emit hard line breaks when the flag is set. Additionally, the `code` parser (only in Markdown reader) handles code elements spanning across several lines correctly.