refactoring of BlockParser#select_leaf_type()
removed unused constant HEAD_RE defined in BlockParser
refactoring of BlockParser.assign_head_re()
renamed BlockParser::LEAF_HEAD_PATS BlockParser::LEAF_HEAD_PAT
BlockParser.assign_head_re() now returns the values for LEAF_HEAD_PAT, NOT_PARAGRAPH_LEAF_TYPES and NUMBER_OF_NOT_PARAGRAPH_LEAF_TYPES