parse.py: Update docstrings
parse.py: Introduce tag header, ditch Parser.line state
parse.py: Reintroduce Parser.line as Parser.single_line_literal, fix pipe tag
parse.py: Treat tag concluder = as if it's a tag
parse.py: Let Parser.verbatim skip leading whitespaces
parse.py: Fix trailing ) appearing as part of attribute value
parse.py: Fix reuse of Tag.attr